US President Barack Obama has said one of the main concerns in the rebuilding effort in the aftermath of Superstorm Sandy is to restore electricity for millions on the east coast of the country.

"Our biggest priority right now is getting power turned back on," Mr Obama said after surveying the storm damage.

"We are here for you and we will not forget, we will follow up to make sure you get all the help you need until you rebuild."

He met with rescue workers and residents in Atlantic City in New Jersey, and toured the wreckage aboard presidential Marine One helicopter alongside Governor Chris Christie, a top supporter for Republican presidential challenger Mitt Romney.

While urging patience, he cautioned that problems from the storm would not be solved "overnight," and that the federal government will work with state and local officials closely in the clean-up efforts.

President Obama has put campaign travel on hold this week due to focus on storm response.

Financial markets reopened with the New York Stock Exchange running on generator power after the first weather-related two-day closure since an 1888 blizzard.

Packed buses took commuters to work with New York's subway system halted after seawater flooded its tunnels.

Sandy crashed ashore with 130km/h winds on Monday as a rare hybrid superstorm after merging with another system. It was the largest storm by area to hit the United States in generations, after killing dozens of people as a hurricane in the Caribbean.

It is likely to rank as one of the costliest storms in US history. One disaster-modelling firm said Sandy may have caused up to $15bn in insured losses.

About six million homes and businesses in 15 US states remained without power today, down from a high of nearly 8.5 million, which surpassed the record 8.4 million customers who went dark from last year's Hurricane Irene.

While markets reopened, floodwaters receded and residents went back to work by car, bicycle and bus in New York. Damage forced evacuation of Bellevue Hospital, known for psychiatric and emergency care.

Five hundred patients were being moved, Mayor Michael Bloomberg said. Evacuations of four other hospitals and 17 chronic care facilities had already been ordered.

An evacuation order for 375,000 New Yorkers in low-lying areas remained in effect, and with subways down, the mayor said cars must have at least three passengers to enter Manhattan.

Across the Hudson River in Hoboken, New Jersey, floodwater that reached chest high on Monday was still knee high this morning.

The New York area's John F. Kennedy and Newark airports reopened with limited service after thousands of flights were cancelled, leaving passengers stuck for days. LaGuardia, a third major airport, was flooded and closed, but is scheduled to reopen tomorrow.

Limited New York subway service is due to return tomorrow, four days after the system, with daily traffic of about 5.5 million people, shut down ahead of the storm. Some commuter rail service is due to come back on line tonight.

A number of scheduled flights into and out of Dublin Airport have been affected by Sandy.

Aer Lingus plans to operate an additional service between Dublin and New York tomorrow.

The additional flight will depart from Dublin at 1pm.

Customers who had bookings for cancelled flights are asked to visit the airline's website and the ''Manage Booking'' facility to book onto the extra flight.

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