The four "most solid" buildings on Saint Martin, shared by France and the Netherlands, were destroyed, French Interior Minister Gérard Collomb said.

Communications between Paris and Saint Martin and Saint Barthélemy are down.

The category five hurricane, the highest possible level, has sustained wind speeds reaching 300km/h (185mph).

The most powerful Atlantic storm in a decade first hit Antigua and Barbuda, before moving on to Saint Martin and Saint Barthélemy (also known as St Barts).

It is expected to move on towards Puerto Rico and the Dominican Republic. In the US, Florida's Key West area has ordered a mandatory evacuation, with landfall expected at the weekend.

The French government said earlier it was worried about thousands of people who had refused to seek shelter on the islands.

In a statement, the interior minister said France was sending three emergency teams to the islands, two from France and one from Guadeloupe.

"The winds are going to turn in the other direction and there's a risk of further damage," Mr Collomb said.

There was no news of possible casualties, he said, adding that "more rustic structures" had "probably been completely or partially destroyed".

French Overseas Territories Minister Annick Girardin said the hurricane had caused major flooding in low-lying areas, and coastlines were being "battered extremely violently" by the sea.

Some 40,000 people live in the French part of Saint Martin, with around the same number estimated to live on the Dutch side. About 9,000 people live on Saint Barthélemy.

'We don't know'

Local reports suggest Antigua escaped major damage but the situation on Barbuda is unclear. The Antigua Observer said it had received initial reports of roofs being blown off on both islands.

"We really don't know what's happening," Antigua radio journalist Gemma Handy told the BBC.

"We're trying to be positive, but nobody's really going to relax over here. It's a very close-knit community of just 90,000 people. Most people live here on Antigua, there's about 2,000 on Barbuda, so lots of people right now are waiting very anxiously for word from their relatives over there."

Thousands of people have been evacuated from at-risk areas across the Caribbean. Residents have flocked to shops for food, water, and emergency supplies.

Airports have closed on several islands which are popular holiday destinations, and authorities have urged people to go to public shelters.

US President Donald Trump has declared a state of emergency for Florida, Puerto Rico and the US Virgin Islands, mobilising federal disaster relief efforts.

In Florida's Key West, visitors will be required to leave on Wednesday morning, with residents due to follow in the evening.

"Watching Hurricane closely," Mr Trump tweeted on Wednesday. "My team, which has done, and is doing, such a good job in Texas, is already in Florida. No rest for the weary!"

Parts of Texas and Louisiana are dealing with the damage done by Hurricane Harvey in late August. But it is not yet clear what impact Hurricane Irma might have on the US mainland.

A third tropical storm, Jose, has formed further out in the Atlantic behind Irma, and is expected to become a hurricane later on Wednesday, according to the US National Hurricane Center.

Seeing multiple storms developing in the same area of the Atlantic in close succession is not uncommon.

Rarer though is the strength of the hurricanes, with Harvey expected to make landfall in the US as a category four.

There have never been two category four storms making landfall on the US mainland during the same season, since records began.