MIAMI, Florida (CNN) -- Hurricane Omar strengthened early Thursday as it blasted the northern Leeward and Virgin Islands with powerful winds and inundating rains.
Omar's maximum sustained winds increased to 125 mph -- up from 115 mph overnight, the National Hurricane Center said.
"Preparations to protect life and property should be rushed to completion," the hurricane center said.
Omar strengthened into a Category 3 storm Wednesday night as it raced to the northeast at about 25 mph.
Earlier, the 500,000 barrel-a-day oil refinery Hovensa, on St. Croix, was shutting down all equipment "except those necessary to maintain power supply in the complex," refinery spokesman Alex Moorhead said in a written statement.
The move was to ensure the safety of employees and the operation of the refinery, jointly owned by Hess Corp. and Venezuela's state oil company.
The U.S. Coast Guard closed the Christiansted Harbor, where the refinery is located, on Tuesday. It will remain closed, Moorhead said, until the hurricane passes and all facilities are checked to be in good order.
As of 5 a.m. ET, the storm was about 70 miles (110 kilometers) north of St. Martin, forecasters said. It has already dumped heavy rain on the U.S. and British Virgin Islands.
Puerto Rico and some portions of the northern Leeward Islands, which includes the Virgin Islands, could get up to 20 inches of rain, according to the forecast.
"These rains could produce life-threatening flash floods and mudslides," the hurricane center warned. The storm could dump up to 8 inches of rain over the Netherlands Antilles, the chain of Caribbean islands off Venezuela's coast. Some areas could receive a foot of rain.
Portions of Venezuela and the northern Guajira Peninsula, which stretches from Colombia and Venezuela into the Caribbean, could receive 6 inches of rain, the NHC said.
The storm could also produce large swells along the west- and south-facing coasts of the Lesser Antilles -- the Caribbean islands that stretch from the Virgin Islands southward to the islands off Venezuela's coast.
The swells could erode beaches and damage coastal structures, the hurricane center said.
A hurricane warning -- meaning winds of 74 mph and higher are expected within a day -- is in effect for several Leeward Islands, including U.S. and British Virgin Islands, St. Martin, Saba, St. Eustatius, St. Barthelemy and Anguilla.
Puerto Rico is under a tropical storm warning and a hurricane watch, as are St. Kitts and Nevis, the NHC said.
Antigua, Barbuda and Montserrat are under a tropical storm warning, meaning they could experience tropical storm conditions over the coming 24 hours. A tropical storm watch is in effect for Guadeloupe.
The storm's forecast track shows it heading into the open waters of the Atlantic Ocean after crossing over the Virgin Islands and sweeping past Puerto Rico, but hurricane tracks are subject to variation, and such long-range predictions can change.
Omar formed Tuesday in the eastern Caribbean. It is the 15th named storm of the Atlantic hurricane season, which began June 1 and ends November 30.