Hurricane Irma pounded Cuba's northern coast today and barrelled toward Florida's Gulf Coast as authorities scrambled to complete an unprecedented evacuation of millions of residents hours before the storm lashes the state.

Irma, one of the fiercest Atlantic storms in a century, is expected to rip through the Florida Keys archipelago tomorrow morning.

It will make landfall on the Florida peninsula somewhere west of Miami on a track that would take it up the state's west coast, including Tampa, forecasters said.

Officials in Florida have ordered a total of 6.3 million people, or about a third of the state's population, to evacuate, creating massive traffic jams on highways and bringing huge crowds to shelters set up around the state.

Hurricane Irma Cuba
Strong waves hit the pier in Havana, Cuba, this afternoon
Zoo Miami
A group of flamingos and spoonbills stay inside a hurricane-resistant building at the zoo in Miami

Irma, which has killed at least 22 people in the Caribbean, is likely to inflict billions of dollars in damage in one of the most populous and fastest-growing states.

It could bring winds in excess of 100 miles per hour and a storm surge up to 4.6 metres high along the state's west coast, which is expected to trigger flooding.

The hurricane could rival any storm in Florida's history, with the storm surge generating flooding along the entire west coast, Governor Rick Scott said.

"Think about that, 15 feet is devastating and will cover your house," he said at a news conference today.

Irma, located south of Miami, still ranked as a Category 5 storm when it crashed into Cuba during the morning. It gradually weakened to a Category 3 storm as it bumped along the island's northern coastline, downing power lines, bending palm trees and sending waves crashing over sea walls.

Maximum sustained winds dipped to around 201km/h today, the US National Hurricane Center (NHC) said.

But Irma is expected to regain strength as it steams over warm waters south of Florida, according to the NHC. It was

unclear if it would regain Category 4 or even Category 5 status, the highest ranking possible.

On Florida's west coast, a long line of people in Estero, north of Naples, lined up to enter an arena that officials converted into an evacuation shelter, one of hundreds that have opened up across the state.

To the east, Luise Campana Read was one of those who chose to ignore warnings and stay in her home. She said by phone she planned to ride out the storm in her beachfront condo in Fort Lauderdale, with her elderly mother and other family members.

"With a 97-year-old, there was no way I was going to have her sleep on a cot or a blow-up mattress" in a shelter, she said.

Later, some communities on Florida's east coast lifted evacuation orders when Irma's projected path edged west.

In Cuba, the destruction along the Caribbean country's north central coast was similar to that seen on other Caribbean islands over the last week as Irma plowed into Ciego de Avila province around midnight.

It was the first time the eye of a Category 5 storm had made landfall since 1932, state media said, and the island's Communist government ordered the evacuation of more than one million people, with most sheltering with family and friends.

In Palm Beach, US President Donald Trump's waterfront Mar-a-Lago estate was under evacuation order.

The outer band of Irma was already whipping south Florida with winds of more than 48km/h this evening, and the governor said thousands of people in the state were without power.

About nine million people in Florida may lose power, some for weeks, said Florida Power & Light Co, which serves almost half of the state's 20.6 million residents.

The window for people in evacuation zones to flee was drawing to a close today, officials said, warning as petrol stations would soon be without fuel and bridges would be closed in some areas.

"You can't play chicken with this thing or try to outrun the storm," US Senator Marco Rubio of Florida said at a news conference.

"The last thing you want to be is out there, on the road, trying to get away from the storm, as this thing is zig-zagging around."

Irma was set to hit the United States two weeks after Hurricane Harvey, a Category 4 storm, struck Texas, killing about 60 people and causing property damage estimated at up to $180bn in Texas and Louisiana. Officials were preparing a massive response, the head of FEMA said.