The first hurricane of the Atlantic season weakened slightly early on Friday to a category two storm, with winds of up to 110mph (175km/h).
Mandatory evacuations have been ordered in parts of Delaware, Maryland, New Jersey and North Carolina.
Irene, now leaving the Bahamas, has already caused havoc in the Caribbean.
States of emergency have been declared in North Carolina, Maryland, Virginia, Delaware, New Jersey and New York.
President Barack Obama also declared an emergency in North Carolina, where Irene is due to make landfall first, on Saturday afternoon.
The move allows greater co-ordination between state and US federal disaster management authorities.
“In this emergency I am activating all levels of state government to prepare for any situation that may be caused by Hurricane Irene,” said New York Governor Andrew Cuomo.
In Washington DC, which is under a tropical storm watch, Sunday’s scheduled dedication of the newly opened memorial for Martin Luther King Jr – which President Barack Obama had been expected to attend – has been postponed until at least September.
Huge wind span
At 20:00 EDT on Thursday (00:00 GMT on Friday), Irene was gradually moving away from Abaco Island in the Bahamas, and remained 550 miles south south-west of Cape Hatteras, North Carolina, said the US National Hurricane Center.
The huge storm boasts hurricane force winds extending 80 miles from its centre, and tropical storm winds reaching up to 295 miles from the eye.
Forecasters said Irene could strengthen slightly throughout Friday ahead of its expected arrival in North Carolina on Saturday.
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It is then expected to weaken as it moves up the east coast, diminishing in strength by Sunday.
US authorities are warning of dangerous storm-surge seas, high waves and rip-tide currents along the coast stretching up from North Carolina, through Chesapeake Bay and Maryland’s Eastern Shore.
Further north still, New Jersey, New York City and Long Island – densely populated areas not usually prepared for hurricanes – are expected to take a buffeting.
Safety ‘not guaranteed’
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 air travel at the weekend.
In Virginia, the US Navy ordered its Second Fleet to leave Norfolk Naval Station in Virginia on Thursday morning and head out to sea.
“The forecasted destructive winds and tidal surge is too great to keep the ships in port,” said Vice Adm Daniel Holloway, the fleet’s commander.
“There is a much greater potential of not only the ships being damaged, but also the pier infrastructure.
“Having the ships under way also makes them ready and available to respond to any national tasking, including any needed disaster response efforts.”
North Carolina emergency officials have extended evacuation orders to include more than 200,000 tourists and residents in three coastal counties.
Visitors to the region have been leaving the area, while residents are preparing to ride out the storm by stocking up on food, water and fuel.
“Businesses are boarding up. Nobody can guarantee their safety,” Dare County emergency management spokeswoman Sharon Sullivan told the Associated Press.
New Jersey Governor Chris Christie warned would-be holiday makers to avoid the shore, and urged people to evacuate ahead of the storm’s anticipated arrival on Saturday night.
In New York City, more than 1,000 miles north of the storm’s location on Thursday morning, Mayor Michael Bloomberg urged residents of low-lying and beach-front areas to find a place to stay on higher ground ahead of Irene’s anticipated arrival on Sunday.
The New York Police Department, the nation’s largest, moved 50 small boats to low-lying areas to be ready for rescue missions.
The city’s social services agencies are doubling their efforts to ensure homeless New Yorkers have access to shelter.
Crews were also clearing the city’s drainage system to make room for the torrents of rain expected when the storm passes to the city’s east.
The exact track of the hurricane is uncertain, but US emergency officials said the east coast from the Carolinas to New England was preparing for its impact.
“This is going to be a big storm,” said Craig Fugate, the head of the US Federal Emergency Management Agency. “Just because it hits one area doesn’t mean it’s not going to cause damage further up the coast.”