Hurricane Omar pounded the Virgin Islands with lashing rain and 120 mile-per-hour winds on Thursday after strengthening into a major category three storm and taking aim at Puerto Rico.
Officials warned of possible fatalities with forecasters saying Omar could rain up to 20 inches (50 centimeters) as it churned across the Northern Leeward Islands and the Virgin Islands.
"These rains could produce life-threatening flash floods and mudslides," read a warning from the National Hurricane Center (NHC) in the United States.
In Puerto Rico all commercial flights and public events were canceled and schools and shipping lanes were closed by order of Governor Anibal Acevedo Vila, as the island made anxious preparations for the impending storm.
Rescue operators set up 18 shelters in the eastern part of the island that was expected to receive the brunt of the storm.
Omar sent the US and British Virgin Islands into lock-down and was "crossing the Northern Leeward Islands with 120 mile-per-hour (193 kilometer-per-hour) winds," the hurricane center said.
The storm upgraded late on Wednesday to a category three on the Saffir-Simpson scale of one to five which measures the potential damage and flooding a hurricane might cause upon landfall.
Particular areas of concern were the coastlines of Puerto Rico and the Virgin Islands, where massive wave surges of four to six feet (1.2-1.8 meters) above normal were predicted.
"Omar is expected to produce large swells that will affect the west- and south-facing coasts of Puerto Rico and the islands of the Lesser Antilles," the NHC said. "These swells could cause beach erosion and damage to coastal structures."
The center of the hurricane was 25 miles (45 kilometers) east of Saint Croix in the US Virgin Islands at 0500 GMT.
The core of the storm was headed on a northeasterly track and was expected to pass between the Virgin Islands and the Northern Leeward Islands in the next couple of hours, the NHC said.
Hurricane warnings were in effect in the US Virgin Islands and the outlying Puerto Rican islands of Vieques and Culebra, as well as Saint Martin/Marteen, Saba, Saint Eustatius, Saint Barthelemy, the British Virgin Islands and Anguilla.
Saint Martin/Marteen, which is divided between France and the Netherlands, hosts the Caribbean's main air hub after Puerto Rico.
The busy 2008 hurricane season has included devastating Hurricanes Gustav and Ike, which caused millions of dollars in damage in Haiti, Cuba and the United States.
Hurricanes and tropical storms have killed hundreds across the Caribbean and in Mexico, with Haiti the poorest nation in the Western hemisphere being the worst hit.