HURRICANE Irma is lashing southwest Florida with the deadly storm threatening tornadoes on the state's east coast.

Irma has lost some strength as it travels north along Florida's west coast, and is now a Category 2 hurricane with sustained winds of 177km/h.

But forecasts warn it will remain a powerful storm as it flooded Miami streets and knocked out power to about 2.3 million homes and businesses.

The cities and towns of Fort Myers and Tampa are now braced to feel the full effects of the storm in a few hours, after Naples was hit.

"Pray, pray for everybody in Florida," Governor Rick Scott said on Fox News Sunday as some 116,000 people statewide took refuge in shelters.

There are fears Tampa is unprepared for the massive wind, rain and storm surge headed its way.

All of southern Florida was feeling the storm's effects on Sunday afternoon, with at least three killed, a woman forced to deliver her own baby and trees and apartment towers swaying in high winds.

US President Donald Trump has declared a major disaster in Florida, and ordered federal funds to help the state and NGO recovery work that will begin once the storm allows.

He called the storm "some big monster" as it battered the Florida coast.

"The bad news is that this is some big monster," Trump said at the White House.

"Right now, we are worried about lives, not cost," he said after returning from Camp David.

Earlier, the National Hurricane Center said the storm had maximum sustained winds of 195km/h, dropping it to a Category 3, the midpoint of the five-step Saffir-Simpson scale.

Irma had been one of the most powerful hurricanes ever seen in the Atlantic, killing 28 people in the Caribbean and pummelling Cuba with 11m, waves on Sunday.

Its arrival in the US forced one of the largest evacuations in American history.

"The biggest thing you can do now is pray," Governor Rick Scott said.

MAKES LANDFALL IN NAPLES AND MARCO ISLAND

The eye of Hurricane Irma was nearing the city of Naples in South Florida on Sunday afternoon local time.

It has already made landfall on nearby Marco Island as a Category 3 hurricane. A 210 km/h wind gust was recorded at Marco Island Emergency Operations Center at about 3pm local time.

The National Hurricane Center said Irma had winds of 195km/h and was centred 30km south of Naples on Sunday afternoon. It was moving north at 19km/h. At that rate, the centre of the storm should come ashore sometime between 4 and 5pm local time.

A storm surge of 2.7m to 4.5m above ground level was reported there, according to the New York Times.

Irma had been one of the most powerful hurricanes ever seen in the Atlantic, killing 28 people in the Caribbean and pummelling Cuba with 11m, waves on Sunday.

Its arrival in the US forced one of the largest evacuations in American history.

"The biggest thing you can do now is pray," Governor Rick Scott said on Sunday afternoon.

It has already made landfall on nearby Marco Island as a Category 3 hurricane. A 210 km/h wind gust was recorded at Marco Island Emergency Operations Center at about 3pm local time.

The National Hurricane Center said Irma had winds of 195km/h and was centred 30km south of Naples on Sunday afternoon. It was moving north at 19km/h. At that rate, the centre of the storm should come ashore sometime between 4 and 5pm local time.

A storm surge of 2.7m to 4.5m above ground level was reported there, according to the New York Times.

The storm has pushed water out of a bay in Tampa, but forecasters are telling people not to venture out there, because it's going to return with a potentially deadly vengeance.

Fears grew how Tampa would deal with the storm after a July analysis by the Washington Post concluded that "the area is due for a major hurricane, and is not prepared. If a big one scores a direct hit, the damage would likely surpass Katrina."

According to the report Tampa the most vulnerable city in United States to a hurricane.

Tampa's mayor, Bob Buckhorn, declared indefinite curfew in Florida city starting at 6pm local time.

Mr Buckhorn delivered a stark warning online for everyone in one of Florida's most densely populated regions.
Buckhorn has announced a 6pm mandatory curfew.

"We know we are ground zero for this storm. We have avoided it for 90 winds but our time has come to be ready," he wrote on Twitter.

"We are about to get punched in the face by this storm."

On Bayshore Boulevard in Tampa, approximately 100 people were walking Sunday afternoon on what was Old Tampa Bay - a body of water near downtown.

Hurricane Irma's winds and low tide have pushed the water unusually far from its normal position. Some people are venturing as far as 180m out to get to the water's new edge. The water is normally about 1m to 1.5m deep and reaches a seawall.

MIAMI BATTERED AS CRANES COLLAPSE

While residents on Florida's west coast are bracing themselves for the devastating eye of the hurricane, which could generate 4.5m walls of water, the east coast is being threatened by a series of twisters.

Indeed, a "fast-moving tornado" has been reported to be seen on the ground at Fort Lauderdale International Airport, The Sun reports.

In Miami the storm downed two cranes in the city's downtown with social media footage capturing the terrifying moment one of them toppled.

The city - given a last-moment reprieve from the worst of the storm - is suffering major electricity cuts as winds while the downtown area of the city is submerged with flood waters.

Waves poured over a Miami seawall, flooding streets a couple of blocks from the waterfront through the financial district and past consulates, leaving high rise apartment buildings standing like islands in the flood.

POLICE SHERIFF AMONG DEAD

The storm has so far claimed three confirmed victims.

A Florida Sheriff's deputy helping out at a shelter and a corrections officer on his way to work were killed in a head-on crash, according to the Miami Herald.

Hardee County Sheriff's deputy Julie Bridges, a mum of one and officer for the past 18 years, crashed at 6.30am Sunday at Zolfo Springs, about 100km from Sarasota. Sergeant Joseph Ossman was killed as he drove in to start his shift at Hardee County Correctional Institute.

Another man was killed after winds caused him to lose control of the truck he was driving through Monroe County, which contains Key West. He had been carrying a generator, local officials told ABC News.

IRMA smashes Florida: 'Everything is under water'

THREE people were killed in car crashes, caught in Hurricane Irma's destructive path as it made landfall in Florida, according to US officials.

Announcing itself with roaring winds, Irma swept over the Florida Keys for the start of what could be a slow, ruinous march up the state's west coast towards the heavily populated Tampa-St Petersburg area.

As winds pushed a wall of water up to 4.5 metres high, Florida Governor Rick Scott implored: "Pray for us."

The hurricane is expected to rake Florida's west coast throughout Sunday, a change from earlier predictions that left some residents and officials scrambling to find shelter.

At least 1.4 million customers are without power, including more than 500,000 in Miami-Dade County, reports Reuters.

In Miami the storm downed a huge crane in the city's downtown with social media footage capturing the terrifying moment. Whitecaps were also seen forming on streets in the Brickell neighbourhood of Miami as the storm surge rushed into the city.

Earlier, the eye of the Category 4 storm hit Key West before being revised to Category 3, bringing winds of up to 210km/h and threatening dangerous storm surges. The eyeball is where the most severe weather occurs while the eye of the storm brings calmer conditions.

The National Hurricane Centre announced the arrival of the eyeball in their 7am advisory local time.

"People ask what they can do for us," Mr Scott said on Fox News Sunday. "Pray for us. We need volunteers, nurses. … I hope everybody will pray for us."

Mr Scott warned the devastating storm surge from Irma will cause the waters to rise quickly and overwhelm everything in its path.

"This water is going to come in very quickly, it's going to cover your first floor potentially or more and then eventually it's going to come out. I don't know how you're going to survive that," he said.

With first responders unable to hit the streets because of the lashing wind and rain, Mr Scott said residents who haven't evacuated or sought shelter are on their own.

"This is an extremely dangerous and life-threatening situation!" the National Weather Service in Key West said earlier, urging those who had not heeded dire warnings to evacuate to take shelter "now to protect your life!".

Florida's Naples Mayor Bill Barnett said he had never dealt with a storm like this. "This is just a monster," he said.

Flooding of up to 4.5 metres - enough to cover a house - were expected and some areas are already experiencing heavy rain and hurricane-force winds..

A tornado funnel cloud also formed off the coast of Fort Lauderdale on Saturday about 6.40pm local time, with the US National Hurricane Centre warning that "a few" more were possible in south and central Florida.

Earlier the National Weather Service Key West said winds were "imminent" and told residents in the Florida Keys "it is time to hunker down". In a tweet it advised people not to go outside, to get away from windows and to "put your shoes on now!".

The impact of the storm could be felt in the neighbouring state of Georgia with a tropical storm warning issued for Atlanta, the first time that has happened in the city's history. The warning means that tropical storm conditions are expected within the next 36 hours. Peak winds were expected to reach 48 to 64km/h with gusts of up to 88km/h.

Irma regained strength on Sunday as it headed towards southwest Florida after leaving a path of destruction across the Caribbean.

The city of St Petersburg is likely to take a direct hit from the storm as well as Tampa.

Neither St Peterburg or Tampa have suffered a major hurricane in nearly a century.

The New York Times reported that St Petersburg enacted a curfew starting at 5pm, and Tampa's mayor, Bob Buckhorn, announced that a curfew would be in effect starting at 6pm Sunday. "Everybody has a plan until they get punched in the face," he said, paraphrasing Mike Tyson. "Well, we're about to get punched in the face."

Mr Buckhorn said anyone remaining in the city's mandatory evacuation zone had hours to move out, as it faces a potential 4.6m storm surge.

"This is our worst nightmare," he said of the possible looming inundation on CNN.

"We are about to get our own version of what hell looks like."

More than 6.3 million - nearly a third of Florida's population - have been ordered to evacuate.

Earlier, MacDill air force Base, the military installation home to US Central Command, issued mandatory evacuation orders.

At least 25 people have been killed since Irma began its devastating march through the Caribbean earlier this week.

Terrified Cubans who rode out Irma in coastal towns - after the storm made landfall on Friday as a maximum-strength Category 5 storm on the Camaguey archipelago - reported "deafening" winds, uprooted trees and power lines, and blown rooftops.

There were no immediate reports of casualties, but officials reported "significant damage." A total of 1.5 million people were evacuated.

Authorities in Havana were evacuating people from low-lying districts at risk from Atlantic storm surges. Enormous waves lashed the Malecon, the capital's emblematic seafront, causing seawaters to penetrate some 820 feet into the capital, AFP journalists found.

MASS EXODUS

In Florida, cities on both the east and west coasts took on the appearance of ghost towns, as nervous residents heeded insistent evacuation orders.

Irma is so wide that authorities were bracing for destructive storm surges on both coasts and the Keys.

And hurricane-force winds are expected to lash the peninsula as it rolls north toward Georgia.

On highway 75 along the western coast of Florida, a steady stream of cars pressed northward as thousands fled at the last minute.

Strip malls, fast food restaurants and retail giants were all closed for business.

In Key West, police had opened a "shelter of last resort" for those who had ignored mandatory evacuation orders.

Scott Abraham, who lives on the fifth floor of a beachfront apartment building in Miami Beach, is planning to ignore evacuation orders and ride the storm out with his wife and two kids.

"If I lived in a house I would have left, but if it gets flooded here it's going to take a week at least to come back. I don't want that," he said.

Warning that Irma would be worse than Hurricane Andrew - which killed 65 people in 1992 -

Mr Scott said all 20.6 million Floridians should prepare to flee.

Cuban-American Orlando Reyes, 82, was forced to leave his assisted living facility in Miami Beach.

"It is frightening," he told AFP at a shelter in Miami. "We had to leave without a cent, without taking a bath, or bringing anything."

People wait in the cafeteria for their named to be called as a Red Cross volunteer (left) gives out room assignments at a shelter within the Pizzo Elementary School in Tampa, Florida on Saturday. Picture: Jim Watson/AFP

People wait in the cafeteria for their named to be called as a Red Cross volunteer (left) gives out room assignments at a shelter within the Pizzo Elementary School in Tampa, Florida on Saturday. Picture: Jim Watson/AFPSource:AFP

AUSSIES CAUGHT UP IN STORM

Among hundreds who elected to stay out the storm was Key Largo car detailer Phillip Hodes's father Mike, who remained at home despite his family's urgings for him to leave with them.

"My dad's staying down in the Keys. I guess he's crazy. They live in a nice concrete house and it's got impact windows and doors to hopefully keep him safe," said Mr Hodes, 24, who is packed into an Orlando hotel room with his mum, dog Rascal and six others.

"We tried to convince him to leave but he didn't really think it would get bad.

"I hope he's there when we get back."

Irma's predicted path has shifted to the west coast of Florida, likely sparing former Sydney yacht broker David Nichols who chose to stay in his home in Fort Lauderdale, on the southeast side of the state.

Mr Nichols, who has lived in Florida for 40 years and endured many hurricanes including 1992's catastrophic Hurricane Andrew, is still expecting a rough time as he rides out the hurricane with wife Sara, in-laws and the family cat Newton. "The track has veered to the left so it's going to give us a big breather," Mr Nichols told AAP.

"I'm not overconfident.

"We'll still see high tropical, low hurricane force winds.

"It's just about being like a boy scout - be prepared." The Nichols are in a single level home on Fort Lauderdale's New River and they have boarded up most of the windows.

PATH OF DESTRUCTION

The storm smashed through a string of Caribbean islands, beginning with tiny Barbuda on Wednesday, followed by the holiday islands of St Barts and St Martin.

Also affected were the US Virgin Islands, Puerto Rico, the Dominican Republic, Haiti and the Turks and Caicos. The Bahamas were spared Irma's worst.

"Houses are smashed, the airport is out of action," St Barts resident Olivier Toussaint said.

"Upside-down cars are in the cemeteries. Boats are sunk in the marina, shops are destroyed." Another powerful storm, Hurricane Jose, had been heading toward the same string of Caribbean islands Irma has pommeled in recent days, but the area received a welcome reprieve when the storm began to gradually weaken and shift course towards the north.

The deteriorating weather had grounded aircraft and prevented boats from bringing relief supplies to hard-hit islands.

The US military was mobilising thousands of troops and deploying several large ships to aid with evacuations and humanitarian relief, as the air force removed scores of planes from the southern United States.