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President Barack Obama has warned that Hurricane Irene, currently looming off the east coast of the US, could be a "historic" storm.
Seven states from North Carolina to Connecticut have declared emergencies ahead of Irene's arrival.
Mandatory evacuations have been ordered in parts of four states, and in low-lying areas of New York City.
The category two storm has weakened a little and is expected to make landfall with winds of up to 100mph (155km/h).
Irene, which has already caused havoc in the Caribbean, is expected to hit the coast of North Carolina on Saturday before barrelling northwards to Washington and New York City a day later.'Don't delay'
At 14:00 EDT on Friday (18:00 GMT), the storm was 300 miles south-south-west of Cape Hatteras, North Carolina, the US National Hurricane Center said.
Mr Obama, on holiday in Martha's vineyard, an island on the Massachusetts coast, said in a statement to reporters: "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. All of us have to take this storm seriously."
He added: "All indications point to this being a historic hurricane."
The White House later said Mr Obama was returning a day early from his break to Washington to lead the government's response to the storm.NY subway to shut
Irene, the first hurricane of the Atlantic season, could affect up to 65 million people in major cities along the east coast from Washington to Baltimore, Philadelphia, New York and Boston - the most densely populated corridor in America.
"We're going to have damages, we just don't know how bad," Craig Fugate, head of the US Federal Emergency Management Agency, told the Associated Press news agency.
"This is one of the largest populations that will be impacted by one storm at one time."
If it hits New York and New England at category two, it will be the region's strongest storm since Hurricane Bob glanced off Massachusetts in 1991, and Hurricane Gloria, which caused extensive damage to New York City in 1985.
Irene boasts hurricane force winds extending 90 miles from its centre, and tropical storm winds reaching up to 290 miles from the eye.
The American Red Cross said it was preparing dozens of emergency shelters along the east coast.
States of emergency have been declared in North Carolina, Maryland, Virginia, Delaware, New Jersey, New York and Connecticut.
Mandatory evacuations have been ordered in parts of Delaware, Maryland, New Jersey, North Carolina and in the nation's biggest city, New York.
New York Governor Andrew Cuomo said the state's transport network, including the New York City subway, would close from midday on Saturday.
In Washington DC, Sunday's dedication of the new memorial for Martin Luther King Jr - which President Obama had been expected to attend - has been postponed until at least September.
The power company serving the Washington area advised of "potential widespread power outages" at the weekend.
Amtrak, America's passenger rail service, announced it was cancelling train travel south of Washington on the east coast, and airlines predicted widespread disruptions to flights.
Tropical storm-force winds have already begun buffeting North Carolina's coast.
More than 200,000 people are evacuating from coastal parts of the state, while residents hoping to ride out the storm are stocking up on food, water and fuel.
US authorities are warning of dangerous storm-surge seas, high waves and rip-tide currents up the east coast as far as Maryland's Eastern Shore.
In Virginia, the US Navy has ordered its Second Fleet to leave Norfolk Naval Station in Virginia to head out to sea to avert damage to the vessels in port.