A US Airways Boeing 757 carrying 157 passengers was evacuated on Friday at Philadelphia International Airport because of a bomb threat but was later cleared for departure.

FBI special agent J J Klaver said bomb-sniffing dogs found nothing illegal on the aircraft after passengers bound for Anchorage, Alaska, were taken off the flight in what he called "an abundance of caution."

Klaver said a letter had been received in London about the plane, which had earlier flown to Philadelphia from Glasgow, Scotland. A federal official with knowledge of the situation said the letter had claimed there was a bomb on board.

The letter was not received by American officials until after the flight from Glasgow had landed in Philadelphia.

US Airways spokeswoman Liz Landau said the Transportation Security Administration ordered the passengers off the Alaska-bound subsequent flight. It also ordered the aircraft taken to a remote area of the airport to be searched.

The plane was cleared for departure more than two hours after it was evacuated.

Klaver said officials were unsure to whom the letter was sent, whether to the police in London or to the airline. It ended up, however, with Scotland Yard.

"It was never deemed as particularly credible," Klaver said. – Reuters