Dominica has suffered "widespread damage" from Hurricane Maria, Prime Minister Roosevelt Skerrit says, as the storm hammers neighbouring Guadeloupe.
"We have lost all that money can buy," Mr Skerrit said in a Facebook post.
The hurricane suddenly strengthened to a "potentially catastrophic" category five storm, before making landfall on the Caribbean island.
Earlier Mr Skerrit posted live updates as his roof was torn off, saying he was "at the complete mercy" of the storm.
"My greatest fear for the morning is that we will wake to news of serious physical injury and possible deaths as a result of likely landslides triggered by persistent rains," he wrote after being rescued.
Meanwhile officials on the French island of Guadeloupe are warning locals to stay sheltered, even if the hurricane appears to be easing in parts.
Houses and roads have been submerged and half of the rain expected in the region has yet to fall, the Prefect of Guadeloupe, Eric Maire, has warned.
"The phenomenon is still ongoing, it is necessary to remain in a safe place even if lulls appear," Mr Maire said.
Hurricane Maria is moving roughly along the same track as Irma, the hurricane that devastated the region earlier this month.
It has maximum sustained winds of 260km/h (160mph), and was downgraded to a category four after hitting Dominica, before picking up full strength again.
Life-threatening mudslides, flash floods and storm surges have been predicted by the US National Hurricane Center, which monitors the region.
How was Dominica affected?
Dominica, a former British colony with a population of 72,000, is less than 50km long and 25km wide, and the eye of the storm passed directly over it.
It made landfall at 21:00 local time (01:00 GMT Tuesday), and Dominica's PM has called the damage "devastating" and "mind boggling".
"My focus now is in rescuing the trapped and securing medical assistance for the injured," he said, and called on the international community for help. "We will need help, my friend, we will need help of all kinds."
Curtis Matthew, a journalist based in the capital, Roseau, told the BBC that conditions went "very bad, rapidly".
"We still don't know what the impact is going to be when this is all over. But what I can say it does not look good for Dominica as we speak," he said.
All ports and airports are closed and residents near the coast have been ordered to go to authorised shelters.
How did it gather strength so fast?
Maria jumped from a category three to a brutal category five within just a few hours, which was an unexpected shock for people in Dominica.
A factor in its rapid development is that local sea surface temperatures are currently anomalously high by a margin of around one to two degrees, says BBC weather forecaster Steve Cleaton.
The elevated sea surface temperature will have contributed to the rapid development of this system, in concert with other very favourable atmospheric conditions within the locale such as low wind shear, our meteorologist adds.
Montserrat, a British Overseas Territory just to the north of Guadeloupe, is likely to be affected next.
It has been issued with hurricane warnings, along with St Kitts and Nevis, the US Virgin Islands, the British Virgin Islands and Puerto Rico.
Tropical storm warnings have been issued for Antigua, as well as Barbuda, St Martin and Anguilla, all of which are still reeling after Irma.
There is a similar warning for Dutch islands Saba and St Eustatius.
In the British Virgin Islands, British authorities - who have been working on Irma relief projects - are among those who have expressed fears that debris left behind by the first storm could be whipped up by the second, posing an extra threat.
Where has it passed?
As the hurricane struck in the middle of the night it has been difficult to assess the extent of the damage so far.
The French territory of Martinique has been hit by power cuts but is thought to have escaped serious damage.
"Reconnaissance operations are still under way but already we can see that there is no significant damage," said Jacques Witkowski, France's head of civil protection and crisis response, according to Reuters.
AFP news agency said there were reports of flooding, mudslides and power outages in parts of St Lucia.
Both places had their hurricane warnings downgraded to tropical storm watch.
However, in this part of the world, the danger does not always pass when the hurricane moves on. Heavy rains mean mudslides can still be a risk.
Will Irma relief work be affected?
Some islands in Maria's path escaped the worst of Hurricane Irma and have been used as bases to distribute relief to places that were not so fortunate.
Now there are concerns that that this work could be jeopardised if they are badly hit too.
Guadeloupe has been a bridgehead for aid going to Irma-hit French territories, while Puerto Rico - which is expected to be hit later on Tuesday by Maria - has also been offering crucial assistance to its neighbours.
Overseas forces mobilise
Britain, France, the US and the Netherlands all have overseas territories in the Caribbean.
The British government said more than 1,300 troops were staying put in the region and an additional military team had been deployed. A 42-strong military resilience team has also been deployed to the British Virgin Islands.
French Interior Minister Gerard Collomb told AFP that 110 more soldiers would be sent to the region to reinforce about 3,000 people already there.
The Dutch navy tweeted that troops were heading to Saba and St Eustatius to bolster security, after St Martin - an island shared between France and the Netherlands - was hit by looting after the earlier hurricane.
On Monday, President Donald Trump declared a state of emergency for the US territories of Puerto Rico and the US Virgin Islands, where the US military has been evacuating personnel.
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