NEW York mayor Rudolph Giuliani appealed for people to stay calm, saying President George Bush was "on top of it".

Mr Giuliani added: "We asked for air cover. The airports have been closed for a while, just for a precaution.

"We should really focus all of our efforts on trying to see if we can find survivors."

The tunnels linking different boroughs of the city were being closed as a precaution, Mr Giuliani said.

"We are sensible and we understand what happened before, so for a while the city's going to be on an even higher level of security," he said.

Mr Giuliani said the Rockaway area had been hard hit by the World Trade Centre attacks and added: "I passed a church where I have been for I think 10 funerals.

"I have been here probably 20 times for funerals and wakes so the idea that Rockaway could be the victims of this is terrible.

"We will do everything we can to help these people."

He went on: "We should remain absolutely calm. We are just being tested one more time. We will pass this too.

"The firefighters and police are the best in the world at this."

Wreckage from the jet, which appeared to be part of the engine, littered a street some distance from where the plane came down, lending weight to reports that it came apart in mid-air.

Emergency workers cordoned off large sections of the area as they searched for any possible survivors and tried to fight the fire caused by the crash.