Hurricane Irma will have a "truly devastating" impact when it slams into southern coastal areas of the United States, the head of the US emergency agency said on Thursday

"The majority of people along the coast have never experienced a major hurricane like this. It will be truly devastating," Federal Emergency Management Agency (FEMA) chief Brock Long told CNN.

Britain is sending two military vessels to help victims of Hurricane Irma and is earmarking £32 million ($41.8 million, 34.8 million euros) in aid, the government said Thursday.

Hurricane Irma, rampaging across the Caribbean towards the Bahamas and south Florida, is smashing not only homes and hotels but weather records as well.

The French part of the Caribbean island St Martin is "95 percent destroyed" after Hurricane Irma tore through the region, top local official Daniel Gibbs said late Wednesday.

Hurricane Irma will exact a "heavy, cruel" toll on two French Caribbean islands, President Emmanuel Macron said Wednesday after a crisis meeting with key ministers in Paris.

Hurricane Irma has already caused "major damage" on several Caribbean islands, French Overseas Territories Minister Annick Girardin said on Wednesday.

Irma, a still strengthening and dangerous Category Four hurricane, churned toward the Leeward Islands early Tuesday, sparking alarm and alerts from the Caribbean to Florida, which declared an emergency.

Climate change will dominate discussions when the leaders of vulnerable Pacific nations hold their annual meeting in the Samoan capital Apia this week, with global warming threatening their existence, officials say.

Three weeks before elections, Chancellor Angela Merkel will Monday face the toxic "dieselgate" scandal which pits the interests of Germany's powerful auto sector against public health fears over air pollution.

China started emergency monitoring for radiation along its border with North Korea in response to the country's largest nuclear test so far on Sunday.