Britain is sending two military transport planes to the Caribbean on Friday to aid victims of Hurricane Irma as the British Virgin Islands declared a state of emergency.

Two planes loaded with "personnel, supplies and recovery equipment" were to set off on Friday, with one heading to the British Virgin Islands, an archipelago with around 28,000 inhabitants that was subjected to the full force of the mega-storm, a defence ministry spokesman told AFP.

On the ground, governor Gus Jaspert said he had declared a state of emergency, indicating there had reports of both injuries and deaths during the storm. 

"Apart from structural damage, there have sadly been reports of casualties and fatalities," he said in a recorded message to residents, without giving any details. 

In Turks and Caicos, another British territory struck by the hurricane, the disaster agency tweeted: "National shutdown has been declared. All residents & tourists are instructed to stay indoors."

Prime Minister Theresa May on Thursday increased the aid budget for Irma to £32 million ($41.8 million/34.8 million euros) from an initial £12 million and said Britain was also sending two military vessels to assist in relief efforts.

One of the ships, fleet auxiliary vessel Mounts Bay, with Royal Marines and army engineers on board, as well as tents and water purifying equipment was due to arrive in the British Virgin Islands on Friday.

But some islanders criticised Britain's relief efforts as too slow.

Dorothea Hodge, a former government in Anguilla, another self-governing British territory, told the Guardian it was "absolutely disgraceful" it had taken so long for London to respond.

And barrister Josephine Gumbs-Connor, who also lives there, told BBC radio she was "very disappointed".

Foreign office figures show there are currently 88,000 British citizens in the British Overseas Territories of Anguilla, the British Virgin Islands and the Turks and Caicos islands.

Hurricane Irma has been ripping through the Caribbean since Wednesday, with its violent winds and torrential rains leaving a trail of devastation and killing 12 as it hurtles towards the United States where up to a million people were told to flee.