Hurricane Irma howled past Puerto Rico with 185mph winds after reducing the tiny tropical islands of Barbuda and St Martin to rubble and claiming at least eight lives.

The category 5 storm - the strongest hurricane ever recorded in the Atlantic - left a trail of deadly devastation through the Caribbean when it struck on Wednesday on a potential collision course with south Florida. 

Barbuda and St Martin suffered the storm's full fury with roughly 95 per cent of properties destroyed on both islands. Officials said at least six people died on the French part of St Martin - a pristine resort known for its vibrant nightlife.

'It's an enormous catastrophe. Ninety-five percent of the island is destroyed,' top local official Daniel Gibbs said. 'I'm in shock. It's frightening.'

The island, which is divided between the Netherlands and France, was left without drinking water or electricity, and the death toll is expected to rise.

Barbuda, part of the twin island nation of Antigua and Barbuda, also suffered 'absolute devastation' and is 'barely habitable' with more than 90 per cent of dwellings completely destroyed and one child killed. 

This morning, the United Nations said up to 49 million people are in the hurricane's path as aid agencies prepare for a 'major humanitarian response'.

Foreign Secretary Boris Johnson said the UK is 'taking swift action to respond' to the disaster after speaking to the chief minister of Anguilla, a British overseas territory that was among the first islands to be hit. A British naval ship has been deployed to help deal with the aftermath with 40 Royal Marines on board, as well as army engineers and equipment, as authorities struggle to bring aid to smaller islands. 'It is just a total devastation. Barbuda is literally rubble,' Antigua and Barbuda Prime Minister Gaston Browne told ABS TV Radio in Antigua.

He added that the island, which is home to 1,800 people, was left 'barely habitable'. 

'The entire housing stock was damaged... Some have lost whole roofs. Some properties have been totally demolished,' he said. 

'We have estimated the rebuilding efforts to be no less than $150 million. That is conservative, because we're talking about rebuilding everything, all of the institutions, the infrastructure, the telecommunications, the roads... Even the hotels on the island, those are totally demolished, as well. It is terrible.