New York Mayor Michael Bloomberg on Friday ordered an unprecedented mass evacuation and the closure of the subway as millions of Americans along the east coast battened down for Hurricane Irene.

US President Barack Obama cut short his summer vacation and urged Americans in the path of the storm to take immediate action, saying "all indications point to this being a historic hurricane."

The Category Two hurricane, packing winds of 100 miles (155 kilometers) per hour, was set to slam into the coast of North Carolina on Saturday before churning up the eastern seaboard towards Washington, New York and Boston.

The densely populated corridor, home to more than 65 million people, was under the threat of flooding, storm surges, power outages and destruction that experts said could cost up to $12 billion.

Bloomberg told a news conference he had ordered the first-ever mass evacuations from low-lying areas across the Big Apple that are home to some 250,000 people, calling it a "matter of life or death."

"We are issuing a mandatory -- I repeat the word 'mandatory' -- evacuation order," Bloomberg said. The order applies to Coney Island and parts of lower Manhattan, Brooklyn, Queens, the Bronx and Staten Island.

"We have never done a mandatory evacuation before, and we wouldn't be doing this now if we didn't think the storm had the potential to be very serious," he said.

Authorities had earlier announced that New York's massive transit system would begin to shut down midday Saturday in another rare move that could hinder transport into Monday's rush hour.

Heavy rains on ground already soaked from weeks of wet weather could worsen the risk of falling trees, and the wind-driven seawater could swell already high new moon tides, sending a storm surge up the Hudson river that could swamp lower Manhattan and the city's underground train system.

New York state meanwhile said major links into the city would be cut if winds exceeded 60 miles per hour, as predicted.

Authorities have called up 900 National Guard troops and 2,500 power workers to prepare for emergency repair work, the largest ever deployment.

And neighboring New Jersey on Thursday ordered 750,000 people out of the Cape May area.

At 2:00 pm (1800 GMT) Friday, Hurricane Irene was around 300 miles southwest of North Carolina, where tropical storm-force winds were already pummeling the coast, the Miami-based National Hurricane Center said.

It said Irene had weakened slightly and would lose strength once it hit land on Saturday, but would remain a hurricane as it moved up the eastern seaboard, passing over or near the mid-Atlantic Saturday night.

Irene's approach stirred painful memories of Hurricane Katrina, which smashed into the Gulf Coast in 2005, stranding thousands of people in New Orleans and overwhelming poorly-prepared local and federal authorities.

The popular North Carolina beach resort of Kill Devil Hills, one of the first population centers lying in Irene's path, was a ghost town Friday, as forecasters predicted up to six inches (15 centimeters) of rain on the coast.

"We haven't seen a hurricane threat like this in quite a few decades," Chris Vaccaro, a spokesman from the National Weather Service, told AFP.

"This is going to be a very long weekend for the residents of the Mid-Atlantic and the northeast."

Irene will be accompanied by an "extremely dangerous" storm surge that could raise water levels by as much as 3.4 meters (11 feet), the NHC said.

"We're ordering all of our citizens off the coast," North Carolina Governor Bev Perdue said, noting the state could not forcibly eject holdouts but would leave them "at the mercy of themselves and the storm."

Hurricanes are rare in the northeastern United States -- the last major hurricane to hit New York was Gloria in 1985.

Obama has spoken with senior emergency officials and local authorities to coordinate federal help, but he warned people to take immediate precautions ahead of the "extremely dangerous" storm.

"I cannot stress this highly enough. If you are in the projected path of this hurricane, you have to take precautions now. Don't wait. Don't delay. We all hope for the best, but we have to be prepared for the worst," he said.

The US military said up to 101,000 National Guard soldiers were available if needed and designated military bases in three states as staging areas.

Ships with the navy's Second Fleet sailed out of their home port at Hampton Roads, Virginia, to ride out the storm at a safe distance, and aircraft cleared off the tarmac at air force bases in three states.

Chuck Watson, research director at Kinetic Analysis, which does computer modeling of predicted storm damage, estimated $11 or $12 billion from Irene in a "worst-case scenario."

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