The city of New York has ordered the evacuation of low-lying areas in Lower Manhattan, Brooklyn, and Queens over Hurricane Irene described as the worst hurricane on the US East Coast since 1985.

Hurricane Irene was downgraded on Friday to a category 2 storm with winds of 110 mph at its center but it is still feared it could still cause severe and widespread destruction.

Irene is due to hit North Carolina on Saturday, with analysts predicting it could regain momentum.

The order of Michael Bloomberg, the mayor of New York, to clear Brooklyn, Queens and the financial district in Lower Manhattan of people will affect at least 250 000 residents.

"We've never done a mandatory evacuation before and we wouldn't be doing it now if we didn't think this storm had the potential to be very serious," Bloomberg said, as cited by the NYT late Friday.

New York's Metropolitan Transportation Authority, which operates NYC's public transport, including the subway, announced it would shut down the transportation system as of midday on Saturday.

US President Barack Obama has declared a state of emergency in New York, ordering federal aid to supplement state and local response efforts starting on Friday because of uncertainties about Hurricane Irene's potential effects.

"If you are in the projected path of this hurricane, you have to take precautions now," Obama said earlier on Friday on the expected "historic hurricane".

Airlines have cancelled hundreds of flights in areas of the Caribbean already hit by Irene, and serious disruptions are expected in New York and Washington.

US media remind that in August 2005, as Hurricane Katrina was about to hit New Orleans, then-mayor Ray Nagin ordered a mandatory evacuation of the city's 1.3 million residents saying it was likely to overwhelm the city levees but not everyone followed his order.