WASHINGTON: US authorities scrambled fighter jets above US cities on Wednesday as a precaution after a small plane crashed into an apartment building in New York, a top commander said.

Admiral Tim Keating, commander of the US Northern Command, would not say how many cities were under air cover but insisted there was no sign of terrorism in the accident on Manhattan's post Upper East Side.

"Fighters, along with early warning systems, they've been up there for half an hour, 45 minutes," Keating told CNN. 

"There are no indications that there is anything underfoot beyond this one airplane or helicopter, whatever, light civil (aircraft) that flew into this apartment building in New York City," he said.

"But ... we reserve the right to exercise our capabilities, which is what we've done here."