MIAMI - Hurricane Bill, the first of the 2009 Atlantic season, gathered strength and grew into a dangerous Category 4 storm with sustained winds of up to 135 mph on Wednesday, the U.S. National Hurricane Center said.

Bill posed no threat to oil installations in the Gulf of Mexico but authorities in Bermuda, a British territory and reinsurance capital, warned residents to be prepared.

Energy markets watch storms in the Gulf of Mexico closely because the region produces a quarter of U.S. oil and 15 percent of its natural gas.

With winds extending outward 45 miles, Bill was expected to push well past the Leeward Islands late Wednesday and early Thursday but hurricane center officials still urged islanders to be on the alert.

At 5 a.m. EST, Bill's center was about 460 miles (740km) east of the Leeward Islands and moving west-northwest at 16 mph (26 kmh).

The core of this dangerous hurricane will be passing well to the northeast of the northern Leeward Islands late today and early Thursday, the hurricane center advisory said.

The storm had the potential to grow in the next 24 hours and turn toward the northwest, the advisory said.

Hurricanes of Category 3 or higher on the five-step Saffir-Simpson intensity scale are considered major and are the most destructive type.

WATCH ON ANA

Energy traders were keeping an eye on the remnants of Tropical Storm Ana, which was producing thunderstorms over Haiti, Cuba and the Bahamas.

The NHC forecast the storm front had a low chance -- less than 30 percent -- of becoming a tropical cyclone again over the next 48 hours.

One forecaster, AccuWeather.com, said it was unlikely but possible the system could regenerate over the eastern Gulf later in the week. Some forecasters noted Ana had already regenerated once.

Hurricane expert Jeff Masters, founder of the Weather Underground website, said he expected Hurricane Bill to move between Bermuda and the U.S. East Coast toward Canada's Maritime Provinces.

I think the likely main impact (on the U.S. coast) is going to be beach erosion and coastal waves, he said. Direct impacts are unlikely.

Bill will encounter energy-sapping cool water when it reaches North Carolina but could still be a Category 1 hurricane near Nova Scotia and Newfoundland, Masters said. (Writing by Bill Trott; Editing by Matthew Jones)

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