A Singapore Airlines (search) passenger jet from Frankfurt to New York was diverted to Manchester Airport (search) in northern England on Monday after a bomb threat that police said may have been a hoax.

A second flight from Frankfurt, a United Airlines plane en route to Chicago, diverted to Heathrow airport in England on Sunday for mechanical reasons and had been due to resume its journey Monday but was delayed for unspecified security reasons, the airline said.

Singapore Airlines flight SQ26 with 292 passengers and 19 crew landed safely at 6 a.m. without a military escort, Greater Manchester Police said. It was the fourth commercial service diverted as a security precaution in Europe in the last nine days.

The pilot declared a "full emergency" mid-flight, a spokesman for Manchester Airport said. The Boeing 747-400 (search) was bound for New York's JFK airport.

"Early indications are that it could be a hoax, but we cannot confirm that one way or another until full checks are carried out," a spokeswoman for Greater Manchester Police said.

In Germany, federal police spokesman Klaus Ludwig said a bomb threat against the flight was made in a call to police in the southwestern German city of Tuebingen. The same caller also threatened a United Airlines flight to Chicago and a Lufthansa flight to London, he said.

Ludwig said the caller had a male voice but gave no further details.

"We got a security warning from the German security authorities," said Peter Menkel, an official with Singapore Airlines in Frankfurt. "It had nothing to do with a passenger on board — it was external."

A United Airlines spokeswoman said Sunday's UA945 flight from Frankfurt to Chicago was diverted to Heathrow for mechanical reasons.

It had been due to resume its journey on Monday. Police said Monday the flight had been searched and given an all-clear but the airline said it was still being delayed for security reasons. No time had been set for the flight to take off, the airline said.

In Germany, Lufthansa spokeswoman Katrin Haase said the airline had consulted with authorities and deemed the threat against one of its flights was not concrete enough and the flight went ahead.