Updated 9.30pm

Hurricane Irma lost some strength as it pounded southern Florida this afternoon, but forecasts warned it would remain a powerful storm as it flooded Miami streets and knocked out power to about 2 million homes and businesses.

All of southern Florida was feeling the effects of the storm creeping towards the shore, with at least one man killed, a woman forced to deliver her own baby, apartment towers swaying in high winds and trees uprooted.

The National Hurricane Center said the storm had maximum sustained winds of 120 miles per hour (195 kmph) dropping it to a Category 3, the midpoint of the five-step Saffir-Simpson scale.

Tampa and Hurricane Bays saw extraordinarily low tides, with sea life visible and boats grounded, though forecasters warned the storm would soon drive those waters back in with storm surges of up to 15 feet(4.6 m) along the state's western Gulf Coast.

WATCH: Irma roars through Cuba

Small white-capped waves could be seen in flooded streets between Miami office towers.

Irma had been one of the most powerful hurricanes ever seen in the Atlantic, killing 28 people in the Caribbean and pummeling Cuba with 36-foot (11 metre) waves today. Its core was located about 30 miles (48 km) south of Naples by 2pm ET (1800 GMT) and was expected to move along or over Florida's western coast through the afternoon and evening.

Some 6.5 million people, about a third of the state's population, had been ordered to evacuate southern Florida.

"This is a life-threatening situation," Governor Rick Scott told a press conference in which he announced that he had requested a major disaster declaration from President Donald Trump.

"There is a serious threat of significant storm surge flooding along the entire west coast of Florida and 15 feet of impact above ground level in southwest Florida," he said.

Tornadoes were also spotted through the region.

Irma is expected to cause billions of dollars in damage to the third-most-populous US state, a major tourism hub with an economy that generates about 5 per cent of US gross domestic product.

About 2 million Florida homes and businesses had lost power, according to Florida Power & Light and other utilities.

The storm killed 24 as it raged through the Caribbean. It has already claimed at least one life in Florida, a man found dead in his pickup truck, which had crashed into a tree in high winds.

A resident and soldier talk amid scenes of destruction from Hurricane Irma. Photo: Reuters

Buildings sway, roads flooded

The storm winds downed at least one construction crane and shook tall buildings in Miami, which was about 100 miles (160 km) from Irma's core.

Deme Lomas, who owns Miami restaurant Niu Kitchen, said he saw a crane torn apart by winds and dangling from the top of a building.

"We feel the building swaying all the time," Lomas said in a phone interview from his 35th-floor apartment. "It's like being on a ship."

Waves poured over a Miami seawall, flooding streets waist-deep in places around Brickell Avenue which runs a couple of blocks from the waterfront through the financial district and past consulates. High rise apartment buildings were left standing like islands in the flood.

Woman delivers her own baby

One woman in Miami's Little Haiti neighbourhood delivered her own baby because emergency responders were not able to reach her, the city of Miami said on Twitter. The two are now at the hospital, it said.

On Marco Island, right in Irma's path just south of Naples, 67-year-old Kathleen Turner and her husband were riding out the storm on the second floor of a friend's condominium after failing to find a flight out. She feared for her canal-facing home.

"I'm feeling better than being in my house, but I'm worried about my home, about what’s going to happen," Turner said.

Irma comes just days after Hurricane Harvey dumped record-setting rain in Texas, causing unprecedented flooding, killing at least 60 people and an estimated $180 billion in property damage. Almost three months remain in the Atlantic hurricane season, which runs through November.

President Trump spoke to the governors of Alabama, Georgia, South Carolina and Tennessee on Sunday and issued a disaster declaration for Puerto Rico, which was hit by the storm last week, the White House said.

Hurricane Irma is wreaking havoc across Florida. Photo: Reuters