Hurricane Irene: Emergency declared in seven US states
Seven states along the east coast of the US, from North Carolina to Connecticut, have declared emergencies ahead of Hurricane Irene's arrival.
The storm weakened slightly on Friday to category two - with winds of up to 105mph (169km/h) - a strength at which it was expected to make landfall.
Mandatory evacuations have been ordered in parts of Delaware, Maryland, New Jersey and North Carolina.
Irene has caused havoc in the Caribbean and could do the same in the US.
At 11:00 EDT on Friday (15:00 GMT on Friday), the storm was 330 miles south-south-west of Cape Hatteras, North Carolina, the US National Hurricane Center said.
US President Barack Obama, on holiday in Martha's vineyard, an island on the Massachusetts coast, said in a statement to reporters: "All indications point to this being a historic hurricane."
"I cannot stress this highly enough: if you are in the projected path of the hurricane you have to take precautions now," he added.
"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. If you are given an evacuation order, please follow it."
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.
If it hits New York and New England at category two, it will be the region's strongest storm since Hurricane Bob glanced off the Massachusetts coast in 1991, and Hurricane Gloria, which caused extensive damage to New York City in 1985.
"We're going to have damages, we just don't know how bad," Craig Fugate, the 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."
States of emergency have been declared in North Carolina, Maryland, Virginia, Delaware, New Jersey, New York and Connecticut.
President 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.
The American Red Cross said it was preparing dozens of emergency shelters along the east coast.
Forecasters said Irene could strengthen slightly before its expected arrival in North Carolina on Saturday. It is then expected to weaken as it moves up the east coast, diminishing in strength by Sunday.
Heightened waves began hitting North Carolina's Outer Banks early on Friday.
Subway flood fears
More than 200,000 tourists and residents are being evacuated from three coastal counties in North Carolina.
While visitors to the region fled the area, residents have been stocking up on food, water and fuel.
Irene boasts hurricane force winds extending 90 miles from its centre, and tropical storm winds reaching up to 290 miles from the eye.
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 Obama had been expected to attend - has been postponed until at least September.
The power company serving the Washington area warned of "potential widespread power outages" at the weekend.
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 the nation's biggest city, New York - which has not seen a hurricane for decades - hundreds of thousands of people in low-lying and beach-front areas have been advised to move elsewhere ahead of Irene's anticipated arrival on Sunday.
Much of New York's subway system and other infrastructure is underground and could be flooded, officials have noted.
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.