New York has called up 1,000 extra troops from the National Guard as "superstorm" Hurricane Sandy bears down on the city.
Heavy rain and strong winds have started lashing the northeast of the US, as forecasters warn New York could bear the brunt of the one-of-a-kind storm - dubbed a "Frankenstorm".
Sandy is on a collision course with two other weather systems leading to fears it could develop into one of the worst storms on record in the US.
The storm is threatening up to 50 million people on the heavily populated East Coast, and forecasters say it could wreak havoc over 800 miles (1,280km) from the Atlantic coast to the Great Lakes.
Authorities are warning New York could get hit with an 11ft (3.3-metre) wall of water that could swamp parts of lower Manhattan, flood subway tunnels and cripple the network of electrical and communications lines that are vital to the nation's financial centre.
As the hurricane approaches the coast, rescuers have saved 14 crew members of HMS Bounty stranded on lifeboats near the eye of storm - but two remain missing.
:: Live Updates on Hurricane Sandy
Sandy, a Category 1 hurricane, is currently about 380 miles (615km) southeast of New York City, with winds of about 85mph (140kph).
The US National Hurricane Centre (NHC) said it strengthened as it turned toward the coast on a predicted path toward New York, Washington, Baltimore and Philadelphia. It is moving at 15 mph (24 kph).
The centre of the storm was positioned to come ashore Monday night in New Jersey, meaning the worst of the storm surge could be in the northern part of that state and in New York City and on Long Island.
The massive storm, which is hundreds of miles across, is set to stay until at least mid-week, bringing hurricane winds, flooding rains and snow in the Appalachian mountains.
Hundreds of thousands of people have already evacuated coastal areas, but much focus remains on New York.
New York Governor Andrew Cuomo urged residents to use common sense and stay indoors.
"The worst is still coming," he said.
New York and other cities shut down schools and suspended all train, bus and subway services on Sunday night because of the risk of flooding.
Nearly the entire coastline of Staten Island has been evacuated. Parts of lower Manhattan, like Battery Park and Ground Zero, have also been evacuated amid the threat of flooding.
All US stock markets will be closed on Monday and possibly Tuesday, the operator of the New York Stock Exchange said, reversing an earlier plan that would have kept electronic trading going on Monday.
A state of emergency has already been declared in nine states - including New York, New Jersey and Massachusetts.
US President Barack Obama has cancelled a planned campaign appearance in Florida and returned to Washington to oversee the federal government's response to the ever-threatening hurricane.
Mr Obama promised the government would "respond big and respond fast" after the storm hits.
"My message to the governors as well as to the mayors is anything they need, we will be there, and we will cut through red tape. We are not going to get bogged down with a lot of rules," he said.
Federal Emergency management administrator Craig Fugate warned the "time for preparing and talking is about over".
"People need to be acting now," he said.
Airlines have cancelled more than 7,600 flights, with British Airways, Virgin and American Airlines have halted some departures to the eastern coat of the US.
At Heathrow, 47 departures and 12 arrivals have been cancelled.
Sandy was blamed for 66 deaths in the Caribbean before it began travelling northward parallel to the eastern seaboard.