Dominica has suffered "widespread damage" from Hurricane Maria, its Prime Minister says.
The hurricane rapidly strengthened to a "potentially catastrophic" category five storm before making landfall on the Caribbean island.
"We have lost all that money can buy," said Prime Minister Roosevelt Skerrit in a Facebook post.
Earlier Mr Skerrit posted live updates as his own roof was torn off, saying he was "at the complete mercy of the hurricane".
"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.
Maria is moving roughly along the same track as Irma, the hurricane that devastated the region this month.
It currently has maximum sustained winds of 250km/h and has been downgraded to a category four hurricane, but it could increase again as it moves towards Puerto Rico and the Virgin Islands, according to forecasters.
Dominica's PM 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 ... we will need help of all kinds," he said.
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.
The nearby island of Martinique has declared a maximum-level alert while another French island, Guadeloupe, ordered evacuations.
Hurricane warnings have been issued for Guadeloupe, Dominica, St Kitts and Nevis, Montserrat, Martinique, St Lucia and the US and British Virgin Islands, and a hurricane watch is in effect for Puerto Rico, St Martin, St Barts, Saba, St Eustatius and Anguilla.
The NHC said that "a dangerous storm surge accompanied by large and destructive waves will raise water levels by as much as 1.5-2.1m above normal tide levels near where the centre of Maria moves across the Leeward Islands".
It also forecast a maximum potential rainfall of 51cm in some areas of the central and southern Leeward Islands - including Puerto Rico and the US and British Virgin Islands - through to Wednesday night.
"Rainfall on all of these islands could cause life-threatening flash floods and mudslides," it warned.
New Zealander Gareth Butler, who lives on the Turks and Caicos Islands, said the storm was forecast to be category four when it passed within 15-25km of the islands later in the week.
He said there hadn't been time to fully recover from Irma, which had brought "frightening" winds and made a "bit of a mess of the place".
The Puerto Rican government has issued a statement saying it expects the hurricane to make landfall there as a category three on Tuesday.
Virgin Atlantic airline boss Richard Branson, who has a home in the Virgin Islands, has been tweeting ahead of the hurricane's predicted arrival, warning people to stay safe.
In some areas of the British Virgin Islands, entire neighbourhoods were flattened by Hurricane Irma.
Earlier this month, Irma left more than two thirds of homes on the Dutch side of the island of St Martin (known as Sint Maarten) uninhabitable, with no electricity, gas or drinking water.
The French government has said its side of St Martin - known as Saint-Martin - sustained about €1.2 billion in damage, with nine deaths across Saint-Martin and nearby St Barts.