New York Mayor Michael Bloomberg said low-lying sections of the city,  mostly along the city's waterfront, are under mandatory evacuation  orders,

WILMINGTON, North Carolina

Reuters -

The eastern United States ramped up its alert on Friday ahead of Hurricane Irene and New York City ordered evacuations of vulnerable residents as the broad, menacing storm closed in on the Atlantic coast.

As 55 million Americans on the eastern seaboard braced for the weekend onslaught from the nearly 600 mile-wide hurricane, President Barack Obama said its impact could be "extremely dangerous and costly" for a nation that still remembers destructive Hurricane Katrina in 2005.

Hundreds of thousands of residents and vacationers were evacuating from Irene's path, starting in east North Carolina where the hurricane, now packing winds of 100 miles per hour, is expected to make landfall on Saturday.

Tropical storm winds were already arriving along the coast of the Carolinas, the National Hurricane Center said.

A quarter of a million New Yorkers were ordered to leave homes in low-lying areas as authorities prepared for dangerous storm surge and flooding on Sunday in the city and farther east on Long Island.

Some New York hospitals in flood-prone areas were already evacuating patients.

"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," New York Mayor Michael Bloomberg told a news conference.

Those affected were ordered to evacuate by 5 p.m. (2100 GMT) on Saturday afternoon.

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