As Hurricane Irma pushed through the Bahamas and onto Cuba on Friday, officials in the United States ramped up the urgency and depth of their warnings to get to safety before its destructive winds and storm surges arrive there Saturday.

Irma, downgraded to a Category 4 storm, continued its destructive churn through the Caribbean overnight, with Turks and Caicos the latest to be swamped. Farther east, residents in places like Barbuda and St. Martin braced for another hit in the coming days from Hurricane Jose, which was upgraded a Category 4 storm.

Officials are cautioning Florida residents steeled by prior hurricanes to not underestimate Irma, which governor Rick Scott said would be "way bigger than Andrew", referring to the major storm that hit the state in 1992. Brock Long, the Federal Emergency Management Agency administrator, cautioned that everyone in the Southeast United States, from Alabama to North Carolina, should be monitoring the storm and making preparations.

"I can guarantee you that I don't know anybody in Florida who's ever experienced what's about to hit South Florida," Long said Friday.

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The National Hurricane Center said Irma remained "extremely dangerous," with winds of 150 mph, and the Florida Keys were at risk of "life-threatening inundation".

Ferocious winds and rain could hit the Florida Keys by late Saturday, with the hurricane reaching heavily populated areas of South Florida on Sunday morning.

The hurricane battered the Turks and Caicos Islands early Friday as it headed toward the southeastern Bahamas, where 20-foot storm surges are expected. Tourists on Cuba's northern coast were also evacuated.

At least 20 people have died because of the storm: nine in the French Caribbean, four in the United States Virgin Islands, three in Puerto Rico, one on the Dutch side of St. Martin, two in Barbuda and one in Anguilla.

Irma lashed the southern Bahamas on Friday with intense winds and rain, leaving a trail of downed trees and power lines, damaged roofs and scattered debris.

Mayaguana and Inagua were among the first Bahamian islands to feel the impact. "It was very loud, you could hear the debris flying around crashing into buildings," said Marcus Sands, an assistant superintendent with the police in Abraham's Bay, Mayaguana's main settlement.

In the Turks and Caicos, Virginia Clerveaux, the director of Department of Disaster Management and Emergencies, said officials were assessing the effects, which included torn-off roofs, electricity outages and widespread flooding.

Irma is among the strongest hurricanes ever recorded in the Atlantic Ocean and one of the five most forceful storms to hit the Atlantic basin in 82 years, according to the National Hurricane Center. It had been a Category 5 storm, but at 5am Eastern, the centre downgraded it based on the lower intensity of sustained winds.

At a news conference Friday morning, Scott cautioned that "we will quickly run out of good weather to evacuate". He asked gas stations to stay open as long as possible, pledging that employees would have police escorts home, and asked other private business owners to "please be compassionate with your employees as they prepare for this storm".

"This storm is wider than our entire state and is expected to cause major and life-threatening impacts from coast to coast," he said.

A storm-surge warning was in effect across much of South Florida, including the Keys.

Voluntary or mandatory evacuation orders were in place in Miami-Dade County, the Keys and portions of many other counties. "Today is the day to do the right thing for your family and get inland to safety," Scott said.

Eric Silagy, the chief executive officer of Florida Power and Light Co, said in a news conference that expected outages from the hurricane could affect 4.1 million customers, which is about 9 million people. "We are going to see a lot of debris that is going to be flying through the air," he said.

In Miami Beach, workers prepared for the coming storm by boarding up windows Friday morning on many of the iconic Art Deco hotels.

A mandatory evacuation was to begin Saturday on Georgia's Atlantic Coast, governor Nathan Deal said. The Georgia Ports Authority said the ports of Savannah, the fourth-largest container port in the United States, and Brunswick would be closed starting Saturday.

The island of Barbuda — where nearly all buildings were reduced "to rubble", according to Prime Minister Gaston Browne — braced for new hardship, in the form of Hurricane Jose. The Category 4 storm charged toward the island nation of Antigua and Barbuda and neighbouring islands.

Antigua and Barbuda's attorney general and minister of public safety, Steadroy Benjamin, issued a mandatory evacuation order for Barbuda, meaning every citizen will be transported to Antigua.

Jose is expected to make landfall Saturday, bringing winds of up to 150 mph to the islands, threatening those whose homes were lost or damaged by the earlier storm.

"I actually was just visiting family in Barbuda. I'm from New York," Teline Charles, 33, said. "I never experienced anything like that. The roof came off during the storm, and we actually had to leave the house and run into the car until the eye came and then ran for better shelter."

The islands of St. Martin, St. Barthelemy and Anguilla, which were also battered by Irma, were also under a hurricane watch for Jose. John McKendrick, the attorney general of Anguilla, said Thursday that the island had suffered "huge devastation" from the first storm.

Darrell Gumbs, a constable in the Royal Anguilla Police Force, was still answering phones Friday at his police station, despite the fact that the building's roof had been blown off.

Residents were using the time between the two storms to clean up as much as possible, "so we don't have debris flying around," Gumbs said.

Up to 90 per cent of the homes on the island were damaged, fallen trees blocked many roads, cellphone service was interrupted and electrical service was cut. The entire island was still without power midday Thursday, he said.

In addition, the ports and the airport remained closed because of damage. St. Martin was dealing with a similar level of devastation.

Other parts of the Caribbean that braced for Irma's wrath were bypassed by the worst of the storm, including the Dominican Republic and Haiti, which share the island of Hispaniola. Damage from flooding and power outages was reported on the Haitian side.

Three people were killed in Puerto Rico, and around two-thirds of the population lost electricity.

Cuba started evacuating some of the 51,000 tourists visiting the island, particularly 36,000 people at resorts on the northern coast.

Residents of the US Virgin Islands said Friday that the island of St. Thomas, which is the centre of the chain's cruise ship industry, had suffered significant damage from Hurricane Irma. At least four people died in the US territory, the islands' governor said.

The extent of the destruction in much of the territory, including on St. John, is not yet known, because of the lack of phone service and electricity.

The roofs of the government's headquarters building on St. Thomas and the island's only hospital were blown off, and a large number of homes on the island were destroyed or damaged, said Holland Redfield, a former Virgin Islands senator who now works as a radio talk show host.

Tom Price, the US health and human services secretary, said Friday that the hospital was closing and its patients were being transported to facilities on other islands.

Irma, which also ripped apart buildings and boats on the island of St. John, left the island of St. Croix relatively unscathed, said Redfield, who rode out the storm on St. Croix.

The hurricane, which also damaged police and fire stations, comes amid a crippling financial crisis on the Virgin Islands that left the government unable to pay for basic operations earlier this year.

Kenneth E. Mapp, the islands' governor, was scheduled to travel to St. Thomas on Friday to survey damage. On Thursday, he cautioned residents: "Recovery is not going to happen in just weeks or a few months given the level of the devastation."

Discussing climate change as Irma batters the Caribbean is "insensitive", Scott Pruitt, the administrator of the Environmental Protection Agency, said Thursday night.

Speaking to CNN, Pruitt said the agency's focus was on helping people in Florida and elsewhere prepare and protect themselves by ensuring people have access to clean water and fuel. He did not comment on whether he believes warming global temperatures are worsening the impact of storms, as scientists say is happening. But he asserted that now is not the time for that discussion.

"To have any kind of focus on the cause and effect of the storm; versus helping people, or actually facing the effect of the storm, is misplaced," Pruitt said, adding, "to use time and effort to address it at this point is very, very insensitive to this people in Florida."

Pruitt made similar comments in the midst of Hurricane Harvey, which wreaked havoc last week along the Texas coast.