Powerful Hurricane Earl bore down on the U.S. East Coast on Thursday on a path towards North Carolina's barrier islands, which it was expected to lash with dangerous winds and pounding surf.

The U.S. National Hurricane Centre said the massive Category 4 storm on the five-step Saffir-Simpson scale of intensity had strengthened with its top sustained winds reaching 145 mph (230 kph).

The centre of Earl, which was located about 355 miles (575 km) south of Cape Hatteras at 8 a.m. EDT (1 p.m. British time), was expected to pass near North Carolina's Outer Banks on Thursday night before turning gradually towards the north-northeast as it tracked up the East Coast on Friday.

While a direct U.S. landfall was not forecast, Earl was on track to deliver a sidelong blow to the North Carolina coast ahead of the Labour Day holiday weekend marking the end of the summer vacation season.

Folks should not focus on the exact track because this is a very large hurricane, said Hugh Cobb, a meteorologist with the Miami-based hurricane centre.

Hurricane force winds extend out 90 miles (145 km) from the centre, Cobb said. So even if it doesn't make either a direct landfall, or even come within 60 miles (96 km) of the Outer Banks, they're still going to get hurricane force winds just because of the sheer size of the storm.

Large swells roiled the coastline, and experts warned Earl would bring dangerously high seas.

North Carolina officials expanded mandatory evacuation orders across new areas of the state's low-lying barrier islands on Thursday.

The islands jut out into the Atlantic and are frequently smacked by hurricanes and storms. Cobb said Earl was one of the biggest storms to menace the state since Hurricane Floyd killed more than 50 people in North Carolina in 1999.

Watches and warnings were posted along the Atlantic coast for most of North Carolina, Virginia, Maryland, Delaware, New Jersey, New York, Connecticut, Rhode Island and part of Massachusetts, alerting residents that hurricane and tropical storm conditions were possible within 36 to 48 hours.

Though forecasters said Earl was expected to keep off the U.S. Coast, they warned that it would be very near southeastern New England on Friday night and said any westward deviation from the forecast track could bring the core of the storm over the mainland.

'TIME OF ACTION IS NOW'

Federal Emergency Management Agency (FEMA) Administrator Craig Fugate urged residents along the U.S. East Coast from North Carolina northwards to remain alert and to heed evacuation orders if they come.

The time of preparing is over, the time of action is now. Don't wait for the next forecast, Fugate told CNN. We're prepared from North Carolina to Maine, he said.

No storm has threatened such a broad swath of the U.S. shoreline -- the densely populated coast from North Carolina to New England -- since Hurricane Bob in 1991.

Oil refiners and nuclear power plants along the U.S. East Coast were still monitoring Earl on Thursday, and taking some solace from predictions it was expected to weaken slowly before Friday.

Behind Earl, Tropical Storm Gaston, the seventh named storm of the Atlantic hurricane season, was churning west in the central Atlantic on Thursday on a path that could take it into the Caribbean Sea.

Gaston was expected to strengthen gradually over the next 48 hours and could become a hurricane by Sunday or Monday, the National Hurricane Centre said in its latest advisory.

The storm was located about 1,550 miles (2,495 km) east of the Lesser Antilles, moving west at 9 miles per hour (14.5 kph) with maximum sustained winds unchanged from late Wednesday at about 40 mph (64 kph).

Most early computer models show the system tracking in a westerly direction into the Caribbean, but it was too early to tell whether Gaston would enter the energy-rich Gulf of Mexico.

(Additional reporting by Tom Brown and Pascal Fletcher in Miami, and Joe Silha in New York; writing by Tom Brown; Editing by Will Dunham)

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