Travel chaos from Hurricane Sandy dragged into a fourth day with roads still covered in floodwater and debris, New York's subway still closed and flights cancellations reaching nearly 20,000 Wednesday.

Massive cleanup efforts have restored power and most public services to millions caught in Sandy's destructive path across but authorities warned it could be days or weeks before life fully returns to normal.

Airlines are scrambling to reschedule tens of thousands of passengers stranded after the storm shut down air travel across the East Coast for much of Sunday, Monday and Tuesday.

Domestic and international flights resumed at most airports on Tuesday afternoon and at two of New York's main hubs on Wednesday morning, but it will take time for airlines to reposition planes grounded by the storm.

There's also no telling how long it will take for regular flight service to get back to normal since LaGuardia -- a key component of New York's global transportation hub -- was damaged by flooding.

"LaGuardia Airport is closed until further notice," the Port Authority of New York and New Jersey said.

The slow-moving storm grounded 16,330 US flights on Sunday, Monday and Tuesday -- including hundreds to and from international destinations as far afield as Hong Kong.

Another 2,876 were cancelled Wednesday and 529 of Thursday's flights were cancelled, with those numbers expected to grow, according to the online aviation tracking service flightaware.com.

"The availability of mass transit continues to be a factor in cancellations; it will need to be fully operational before airlines can resume full services," Flight Aware chief Daniel Baker said.

Limited service resumed Wednesday for the 8.5 million people who rely on the New York area's commuter networks and some of the city's subway trains will be running on Thursday, state Governor Andrew Cuomo said.

"There will be no service in Manhattan below 34th street because there's no power for that service below 34th," Cuomo said.

Flooded tunnels and stations also caused substantial damage to the system that could keep some routes offline for a while, officials warned.

Heavy traffic has made for slow journeys and New York Mayor Michael Bloomberg announced new rules Wednesday afternoon preventing vehicles with fewer than three people from entering Manhattan over the East River.

"The streets just cannot handle the number of cars that have tried to come in, so I've ordered that the four East River bridges be restricted to high occupancy vehicles only tonight, coming into Manhattan, meaning three or more people," he said.

Bus service in New York has already been largely restored, with the city offering passengers free rides as residents struggle to get around.

"It feels very strange to have the whole system down," said 75-year-old Julie Harmon, waiting for a bus on 42nd Street.

"It's like World War Three over here!" laughed Regina Glick, a real estate broker waiting across the street. "It feels like we're disconnected from the rest of the world."

Public transit systems shut down late Sunday in Washington, Baltimore and Philadelphia resumed normal schedules Wednesday morning after crews inspected tracks and removed debris.

Amtrak's passenger train service remained crippled by the flooding of tunnels under New York's rivers but regular service elsewhere in the region was beginning to resume Wednesday.