The National Hurricane Centre said Irma was maintaining category five strength with sustained winds near 185 mph (295 kph).
Residents and tourists in the region have been told to stay inside – with the French weather office reporting that St Martin and St Barts have now been struck by the storm.
Daniel Gibbs, president of the territorial community of St Martin, told local media: “We have never seen this in Saint-Martin. Even the walls are shaking.”
Communications are down for thousands and attempts to contact the islands have largely been unsuccessful.
Hurricane Irma makes landfall in Barbuda as the Caribbean braces for devastation
One French media outlet received a text message from St Barts Senator Michel Magras which read: ”Hello, I am sorry, I am shocked by the monster that covers us, the island is devastated, it is apocalyptic, a lot of damage, many roofs destroyed. We are currently living the second part of the hurricane.”
The weather station on St Bart’s measured winds of 151mph (244kph) as Irma approached the island – but the monitoring equipment has since been destroyed by the hurricane.
Meanwhile, French interior minister Gerard Collomb says the St Martin government buildings – the most sturdy there on the islands – have been destroyed.
He told reporters: ”We know that the four most solid buildings on the island have been destroyed which means that more rustic structures have probably been completely or partially destroyed.”
The latest images from the NOAA shows Irma bearing down on the Caribbean
We have never seen this in Saint-Martin. Even the walls are shaking
The hurricane is travelling in a west-north-westerly direction at 15mph (24kph) and passed around 40 miles north of Antigua on its way towards Puerto Rico and Florida.
With millions of people already expected to be affected by Hurricane Irma, the US National Hurricane Centre has also issued a warning about tropical storm Jose – which is following Irma’s path and is expected to turn into a hurricane by this evening.
Jose is forecast to hit the Caribbean islands currently being lashed by Irma at the weekend, meaning relief efforts could be thwarted as the region is bombarded with the catastrophic weather.
One Twitter user wrote: “Some of the biggest gusts from the west in the last few minutes. Crashing into the windows, making us both jump! #HurricaneIrma #Antigua.”
A hospital roof in the British Overseas Territory of Anguilla has collapsed
Garfield Burford, the director of news at ABS TV and Radio on the island of Antigua, said: “We are hunkered down and it is very windy … the wind is a major threat.
“So far, some roofs have been blown off.
“It’s very scary … most of the islands are dark so it’s very, very frightening.”
The seas around Barbuda rose more than two metres as the storm devastated the island, prompting major fears of drowning given that the tiny island is only 38 metres above sea level at its highest point.
The Antigua Met Service says the island, which escaped the eye of the hurricane by just a few miles, only suffered “minimal damage”.
A report on the impact of Hurricane Irma read: “there’ve been no reports of deaths so far. Three persons sought medical attention at the Mount St John Medical Centre. There were four minor incidents.
“The preliminary report is indicating that damage across Antigua and Barbuda is minimal.
“There were several reports of roof damage in Antigua – in areas like Crosbies, Fort Road, Clare Hall, Grays Farm and Pigotts – four roofs completely off.”
This image, taken in Saint Martin, shows the ferocious winds hours before Irma is due to hit
Many homes in Antigua and Barbuda are not built on concrete foundations and are very susceptible to wind damage.
The prime minister of Antigua and Barbuda, Gaston Browne, tweeted to say the islands had been “spared the worst” of Irma’s devastation.
He wrote: “Colleagues, the Lord has protected us and we have been spared the worst of Irma.”
The storm is expected to move slowly across the Caribbean over the next few days before hitting Florida at the weekend.
Hurricane Irma hits Caribbean Island of Barbuda
He wrote: “Watching Hurricane closely. My team, which has done, and is doing, such a good job in Texas, is already in Florida. No rest for the weary!
“Hurricane looks like largest ever recorded in the Atlantic!”
Meanwhile, water is already flooding the streets of St Martin, with the former Dutch colony expecting more high winds and heavy rains over the next few hours.
Britons in the Caribbean who are in the storm’s path have been urged to follow the advice of local authorities and any evacuation orders by the Foreign and Commonwealth Office.
Aid agencies are preparing a huge humanitarian response operation to help those affected by the devastating hurricane.
Walter Cotte, the US regional director of the International Committee of the Red Cross, said the forecast was “extremely worrying” and added he was “anticipating major impacts on a number of islands.”
The tiny island of Barbuda, home to less than 2,000 people, is where Irma first made landfall
“One of the main challenges is going to be logistical, given the isolation of some the islands. We need to ensure a reliable channel for relief efforts in the aftermath of the hurricane.”
The Red Cross has released around £100,000 from its Disaster Relief Emergency Fund to Antigua and Barbuda and St Kitts and Nevis, with more funds set to follow.
The US mainland has already been devastated by Hurricane Harvey in recent days, with the government forced to set aside millions to repair the damage.
Officials in Florida have begun ordering evacuations and there are already huge concerns about Florida Keys, a chain of islands at the southern tip of the state that is a tourist hotspot and home to more than 80,000 residents.
Bahamas prime minister Hubert Minnis has said the government will a issue a mandatory evacuation order for six southern islands as authorities would not be able to help people caught in the “potentially catastrophic” conditions there.
Those who live on the islands will be flown to Nassau on the island of New Providence in what will be the biggest hurricane evacuation in the island’s history.
Mr Minnis warned residents that emergency services may not be able to reach them when the storm is at its height between Thursday and Friday.
Meanwhile, people in Guadeloupe have been urged to stay in the safest rooms in their houses with the French overseas being placed on red alert.
Miami residents stock up with essentials as they prepare for Hurricane Irma to hit people
In a statement, he said: “The security forces have advised that the public should be foremost concerned about their safety and well-being and therefore should refrain from being on the streets after 6 pm.“Only essential workers are expected to be outdoors and on the streets. Moreover, we seriously advise, for their own safety and security, that our citizens and residents remain indoors and continue to listen to all official advisories on the hurricane.
The official government agencies will indicate when it is safe to venture outdoors after the passage of the hurricane.
“All of our national security agencies have been fully mobilized and are on highest alert.”