Hurricane Irene bore down on the Bahamas Tuesday as a category one storm, churning on a track that could see it slam the US mainland later in the week.

Forecasters who downgraded Irene, now packing winds of 90 miles (150 kilometers) per hour, said it could still become a major hurricane by Wednesday as it swirls past a series of islands toward the US east coast.

At 0000 GMT, Irene was centered 50 miles (80 kilometers) west-southwest of Grand Turk Island, and blasting tropical storm-force winds outwards of up to 200 miles (320 kilometers), the Miami-based US National Hurricane Center said.

Fed by warm Atlantic waters, the intensifying storm is expected to whip up ocean storm surges as high as 13 feet (nearly four meters) by the time it reaches the central Bahamas.

The storm is expected to strengthen to a major hurricane, reaching category three strength by Wednesday, the US National Hurricane Center said.

The State Department meanwhile issued a travel warning for the Bahamas and the British overseas territory of Turks and Caicos, urging US citizens already to seek shelter.

As Irene churned over the Turks and Caicos, authorities closed airports, and banks and supermarkets shut their doors.

Government spokeswoman Andrea Been said storm surges and high waves would batter the islands into the night Tuesday, while officials said high winds felled power lines and debris littered the streets of Providenciales.

In the Dominican Republic, authorities said more than 11,000 people were evacuated to shelter before the storm winds brushed the island's north coast Monday night.

Quake-ravaged Haiti, which shares the island of Hispaniola with the Dominican Republic, was largely spared by the storm, although some mudslides were reported near the northern coast.

Hurricane warnings were, however, still in effect for parts of the Bahamas, and US officials urged vigilance.

The entire eastern seaboard should be on alert, they stressed, with Craig Fugate, chief of the Federal Emergency Management Agency, urging residents to realize that it's "critical that you take this storm seriously."

National Hurricane Center chief Bill Read said a "very large" Irene would likely approach the coast of South Carolina and North Carolina early Saturday.

"But New England could also be in play for a possible impact from Irene," he warned in a media conference call.

Irene is forecast to approach the US mid-Atlantic coast at a time when it is packed with tourists for one of the final weekends of summer.

And forecasters said that by the time Irene reaches US shores, it is likely to be an even more potent storm -- perhaps as high as a category four on the five-level Saffir-Simpson scale.

"The stakes are high because it would take just a slight shift in the track to the left to make a dramatic change in the impact of the storm in a hugely populated area," said NHC spokesman Dennis Feltgen.

US forecasters said Irene could travel clear up the US Atlantic coast as far north as Delaware by early Sunday.

Up to 10 inches (25 centimeters) of rainfall were expected in the southeastern Bahamas.

Irene ravaged the US territory of Puerto Rico on Monday, and one woman died there after her car was swept away by flooding as she tried to cross a bridge, authorities said.

Some 1,500 Puerto Ricans were dislocated by the storm, and nearly a million were left without electricity. Clean-up was under way after Irene downed trees and caused flooding in residential areas.

After the storm hit, US President Barack Obama issued an emergency declaration for the territory.

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