Hurricane Irma has caused considerable damage on French island territories in the Caribbean, and casualties are expected, France's president says.
The impact of Irma on St Martin and St Barts would be "hard and cruel", Emmanuel Macron added.
Significant damage is also being reported in the Dutch section of Saint Martin.
The category five hurricane, the highest possible level, has sustained wind speeds reaching 295km/h (185mph).
French Overseas Territories Minister Annick Girardin confirmed at least two people had been killed and another two seriously injured in St Martin and St Barts.
Earlier, Interior Minister Gérard Collomb said the hurricane had caused major floors, and destroyed buildings, including four of the "most solid" buildings on St Martin.
The most powerful Atlantic storm in a decade first hit Antigua and Barbuda, before moving on to Saint Martin and Saint Barthélemy - the French holiday destination popularly known as St Barts.
Its "extremely dangerous core" is forecast by the US National Hurricane Center to move over parts of the northern Virgin Islands on Wednesday, passing near or just north of Puerto Rico, then passing near or just north of the coast of the Dominican Republic on Thursday.
Thousands of people have been evacuated from at-risk areas across the Caribbean. Residents have flocked to shops for food, water, and emergency supplies, and airports have closed on several islands which are popular holiday destinations.
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 the Dutch territory, known as Sint Maarten, the airport has been closed with photos showing debris strewn across the departures area and outside.
There has been a total power blackout, streets are littered with debris, cars are underwater and boats in the ports have been destroyed, Dutch broadcaster NOS reports (in Dutch).
France's interior minister said three emergency teams were being sent to the islands, two from France and one from Guadeloupe.
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 St Barts.
Some islands in the region are almost at sea level and any significant storm surges would be potentially deadly, the BBC's Will Grant reports from Havana.
"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."
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.
The US House of Representatives on Wednesday approved roughly $8bn (£6.1bn) in initial emergency aid for states affected by Harvey. The measure will now go to the Senate.
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 making 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.
Are you in the region? Are you a holidaymaker unable to get a flight home or a resident who has been preparing for Hurricane Irma? If it is safe for you to do so, share your experiences by emailing email@example.com.