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This image provided by the National Oceanic and Atmospheric Administration shows Tomas as a tropical storm early Saturday before it grew to hurricane strength. ((NOAA/Associated Press))

Newly born Hurricane Tomas swept through a cluster of eastern Caribbean islands Saturday, tearing off roofs, damaging houses and downing power lines.

Authorites in St. Vincent were trying to confirm reports that three people died, including two men who might have been blown off a roof, said Jimmy Prince, emergency management spokesman.

Fierce winds tore roofs from nearly 100 homes and more than 400 people sought emergency shelter as the island plunged into darkness, he said.

"Many of them are workers who were unable to get off Mustique," he said, referring to a tiny island just south of St. Vincent.

In St. Lucia, winds also ripped off the roof of a hospital, a school, a stadium and toppled a large concrete cross from the roof of a century-old church, government officials said.

Heavy rains also unleashed a landslide that blocked a main highway linking the capital to the island's southern region.

Prime Minister Stephenson King said he was still stranded in Barbados on an official trip and apologized to people on an island that reported a complete blackout.

"It hurts me to know that I am not around to give courage, strength and guidance at a time when we all must bond together and give support to each other," he said in a statement.

The government ordered two airports and all businesses closed and people called radio stations to admonish parents who were letting children play in the streets, where trees and power lines were falling.

"This is no joke," said calypso singer Nintus, one of the callers.

Organizers of the island's biggest Creole festival called off the event due to the storm, disappointing both would-be revellers and dozens of vendors who travelled to the capital to sell vegetables, fruits and other provisions.

"All my preparations have gone down the drain," said vendor Theckla Darius, from the rural community of Fond Assau. "It's been a lot of effort for nothing."

Also under hurricane warning were St. Vincent and Martinique, where at least 20,000 people were without power, streets flooded and tree branches were down. A cruise ship carrying nearly 2,000 tourists docked instead in Dominica.

Tomas had already knocked down power lines and damaged houses in Barbados as a tropical storm.

Could be Category 3 storm by midweek

The U.S. National Hurricane Center in Miami said Tomas strengthened Saturday night with 150 km/h winds and was centred about 85 kilometres west-southwest of St. Lucia. It was moving west-northwest at 15 km/h.

Tomas, the Atlantic season's 12th hurricane, was expected to drop up to 15 centimetres of rain in the region.

Forecasters said it could become a Category 2 storm Monday evening and possibly reach Category 3 by midweek, with winds around 184 km/h.

It was forecast to head toward Jamaica and could unleash heavy rains in southern portions of the Dominican Republic and Haiti, which is struggling to recover from a devastating earthquake and cope with a recent cholera outbreak.

Haiti issued an orange storm alert, the second highest level. Authorities warned southern and western regions — including the quake-ravaged capital of Port-au-Prince, where an estimated 1.3 million people are living in tent camps — to be on guard for high winds, thunderstorms and possible flooding.

But with few usable storm shelters and no feasible evacuation plan, residents will largely be on their own.