Mandatory evacuations have been ordered in Florida as US authorities urge residents to prepare for “record-breaking” winds after Hurricane Irma claimed two lives in the Caribbean overnight.

The category-5 storm with winds of more than 295 kilometres per hour was one of the most powerful ever recorded in the Atlantic, officials said.

Hurricane warnings were in place for much of the Caribbean, including Puerto Rico, the Dominican Republic, Haiti and the Bahamas, the US National Hurricane Center said.

France’s overseas territories minister said at least two people were killed on St. Martin and St. Barthelemy as Hurricane Irma hammered the islands.

“We’re talking about two dead and two seriously injured for now. Obviously the situation can change very quickly,” Annick Girardin told reporters on Thursday morning (AEST) before boarding a plane for the region.

Ms Girardin said the French government was launching an emergency plan, but it was vital to assess the damage because at this stage authorities could not get access to the worst-hit areas.

The US territory of Puerto Rico declared a state of emergency and residents feared the island’s electrical system was vulnerable to weeks or months of disruption, CNN reported.

Irma had already destroyed several of the strongest buildings on the island of St Martin, according to local media.

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Forced evacuations for some coastal areas in Florida go into effect on Thursday with the Irma expected to make landfall in the US mainland over the weekend.

The evacuation zone includes low-lying areas in Fort Lauderdale, Hollywood, and Pompano and Deerfield beaches.

“Floridians should heed any evacuation order,” Florida Governor Rick Scott said Wednesday.

“(A) storm surge could cover your house. We can rebuild homes – we cannot rebuild your family,” he said.

Florida residents rush for supplies as Irma approaches. Photo: AP

Governor Scott said the approaching hurricane Irma could be more powerful than the strongest storm ever to hit the southern state.

“The storm is bigger, faster and stronger than hurricane Andrew,” Governor Rick Scott said, pointing to a storm that devastated the state 25 years ago in August 1992.

US President Donald Trump added weight to the warnings.

“A new and it seems to be record-breaking hurricane heading right toward Florida, Puerto Rico, and other places,” US President Donald Trump said  Wednesday.

Hurricane looks like largest ever recorded in the Atlantic!

— Donald J. Trump (@realDonaldTrump) September 6, 2017

It comes on the heels of Hurricane Harvey, which developed into a category-4 storm in the Gulf of Mexico before slamming into Texas last month and causing widespread damage in Houston and other areas.

More than 60 people are believed to be dead as a result of Harvey, with the recovery bill expected to reach more than US$180 billion (A$225 billion).

French President Emmanuel Macron said the impact of Hurricane Irma on the islands of Saint Martin and St Barts would be “hard and cruel”.

Significant damage is also being reported in the Dutch section of Saint Martin.

French Interior Minister Gérard Collomb earlier said Irma had caused major floors, and destroyed buildings, including four of the “most solid” buildings on Saint Martin.

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Irma is just the fifth Atlantic hurricane to have maximum sustained winds of 295 kph or greater, according to Phil Klotzbach, a tropical scientist at Colorado State University quoted by the Weather Channel.

– With agencies