Looming Hurricane Earl strengthened and churned up dangerous swells on Wednesday, forcing evacuations on some of North Carolina's barrier islands and prompting storm alerts along much of the U.S. east coast.

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

Earl had sustained winds of 135 mph (215 kph) as it barrelled across the Atlantic, making it a powerful and dangerous Category 4 storm on the five-step Saffir-Simpson scale of intensity, the U.S. National Hurricane Centre said.

While a direct U.S. landfall was not forecast, Earl was on track to deliver a sidelong blow to the North Carolina coast, with its centre seen passing within 100 miles (160 km) of the state's Outer Banks barrier islands late on Thursday or early on Friday. The islands jut out into the Atlantic and are frequently smacked by hurricanes and storms.

Forecasters said a low pressure trough moving out of the southwest over the United States would push Earl to the northeast on Thursday, keeping it off the U.S. Coast. But they warned that any westward deviation from the forecast track could bring the core of the storm over the coast.

If the turns occur a little bit later than we're forecasting it could bring more of a threat at that time to extreme eastern Long Island or southeastern New England, said Hurricane Centre Director Bill Read.

Earl was expected to bring driving rain, high winds, pounding surf and rip currents to the densely populated U.S. coast from North Carolina to New England during the Labour Day holiday weekend marking the end of the summer vacation season.

No storm has threatened such a broad swath of the U.S. shoreline since Hurricane Bob in 1991, said hurricane centre spokesman Dennis Feltgen.

Large swells roiled the coastline, and experts warned Earl would bring dangerously high seas. We could potentially see (storm) surge over 12 feet (3.6 metres) in many areas, and that in fact would cause substantial property damage and property loss, said Howard Botts, a geologist with Corelogic who has close ties to the insurance industry.

EVACUATIONS ORDERED

Earl was a large storm with hurricane-force winds extending 90 miles (150 km) from the centre and tropical storm-force winds extending out 200 miles (325 km).

At 5 p.m. EDT (10 p.m. British time), it was moving northwest through the Atlantic about 630 miles (1,010 km) south-southeast of Cape Hatteras, North Carolina.

Energy companies expected Earl to miss key refineries in New Jersey and Pennsylvania, while nuclear plant operators in North Carolina and Virginia said they did not expect winds severe enough to force shutdowns.

North Carolina Governor Beverly Perdue declared a state of emergency, a move that eased transport restrictions and allowed speedier delivery of essential fuel and gasoline supplies.

North Carolina's Dare County ordered a mandatory evacuation of all visitors on Hatteras Island, a picturesque vacation spot that draws large numbers of tourists each year. Officials said high waves could wash over the coastal highway, stranding those who stay behind.

Vacationers and residents jammed ferries bound for the mainland after evacuations were ordered on Ocracoke Island.

On Hatteras Island, visitor John Gusciora of Leesburg, Virginia, was reluctant to leave the village of Salvo, where he had rented a cottage with three families. The weather was sunny on Wednesday, with little to no breeze.

It's disappointing. We've been coming down here for years and this is the best weather we've had until today. Now we have to try to pack up our bags and beat the rush off the island, Gusciora said. It's sad to go, but it's better to be safe.

It was too early to predict how close the hurricane would come to New York City when it churns offshore east of the city over the weekend, but emergency officials urged local authorities and residents to stay alert.

INSURED LOSSES IN CARIBBEAN

Earl caused $50 million to $150 million (32 million to 97 million pounds) in insured losses in the Virgin Islands, St. Maarten, St. Martin and Puerto Rico when it blew through the northeast Caribbean earlier this week, risk modelling company AIR Worldwide said.

Behind Earl, Tropical Storm Fiona was moving northwest over the open Atlantic, about 140 miles (225 km) north of the Caribbean island of Anguilla on Wednesday, with winds up to 60 mph (96 kph).

Fiona was expected to curve northeast in the Atlantic and keep far away from the U.S. coast while remaining a tropical storm at its peak intensity.

Meanwhile, Tropical Storm Gaston formed in the Atlantic 895 miles (1,440 km) west-southwest of the Cape Verde Islands and was expected to strengthen into a hurricane during the weekend. It was forecast to move west towards the northeast Caribbean islands.

(Writing by Jane Sutton; Additional reporting by Tom Brown and Pascal Fletcher in Miami, and Eileen Moustakis in New York; Editing by Pascal Fletcher and Todd Eastham)

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