Sandy, expected to come ashore later today, could deliver a harsh blow to major cities in its target zone, including New York, Philadelphia, Washington, Baltimore and Boston.

Its centre was forecast to strike the New York-New Jersey area and then move inland towards Philadelphia and the rest of Pennsylvania.

The sheer size of the storm meant its effects would be felt from the mid-Atlantic states to New England.

Officials warned of widespread power outages.

In New York City, subway, bus and train services were suspended yesterday and up to 375000 people were ordered to evacuate low-lying areas.

President Barack Obama, speaking after a briefing at the federal government's storm response centre in Washington, asked residents to heed the orders of state and local authorities.

The storm could bring the country's financial nerve centre to a standstill, although the major Wall Street exchanges said they planned to open as usual today .

New York City schools will be shut today. Other local governments also announced school closures.

Gale-force winds were already starting to buffet Virginia. Tens of millions of people will feel its bluster for as long as two days.

It could be the largest storm to hit the U S , according to the National Oceanic and Atmospheric Administration's website.

More than 700 flights were cancelled yesterday and 2500 more were due to be grounded today .

New Jersey casinos were ordered to close and state officials said bus and rail systems will be shut down today.

Governors in other states put National Guard troops on alert.

"We're just asking people to be patient and be ready for a long haul. But we have a very aggressive power restoration programme in place and I think we're ready," Virginia Governor Robert McDonnell said.

In Washington, Obama said officials had assured him they had all the resources they needed.

He stressed "it is important for us to respond big and to respond fast" to the hurricane's onslaught.

"We're going to cut through red tape and we're not going to get bogged down in a lot of rules," said Obama, who was having to juggle both his re-election bid and his efforts to stay on top of the storm's impact just nine days before election day.