Two people have been killed when a small aircraft crashed into a building in New York City's affluent Upper East Side, police say.
Flames and smoke are coming out of the 50-storey apartment building on Manhattan island.
The FBI says there is no indication that the crash is terrorism-related. A White House spokesman said they were not ruling out any theory.
As a precaution, fighter planes are now flying over a number of US cities.
The New York Fire Department said the aircraft struck the 20th floor of a building on East 72nd Street and York Avenue - identified as the Belaire, a prestigious 50-storey residential tower.
Witnesses told the Associated Press (AP) news agency the crash caused a loud noise, and burning and falling debris was seen.
Emergency workers have rushed to the scene, while firefighters shot streams of water at the flames from the lower floors.
A freelance journalist at the scene, David Cox, added there were "hundreds" of emergency workers on the ground.
"It's just next door to a hospital so there's a large amount of people wearing doctor's coats and various other nurses gear standing round the street," he said.
Another eyewitness told the BBC: ''The plane levelled out and then the next thing I knew it disappeared behind the building and was gone.
"I said I can't believe that this is what I just saw and they kept saying it was a helicopter but it was a plane.''
A woman who lives nearby, Chris Foege, said: "I just stood there in shock, I thought 'this can't be happening to us again'. It was like 9/11 all over again," she told the AFP news agency.
BBC correspondent Guto Harri in Manhattan says there are scenes of chaos on the streets, with fire engines, police cars and ambulances blocking nearby roads.
Are you near the scene of the crash? Send your comments and experiences in using the form below.
You can send pictures and video to: firstname.lastname@example.org
or to send via MMS please dial +44 (0)7725 100 100.
Oh boy... I hope everyone's okay. And I hope that this is not um... 10/11. :uuh: