U.S. airlines stepped up security at British airports Thursday as investigators tried to determine how a bomb got aboard Pan Am Flight 103.

At Heathrow International Airport's busy Terminal 3, used by both Pan Am and TWA, passengers had to put baggage through X-ray machines as part of the new security measures. The baggage was then sealed with tough plastic strips before check-in, to prevent tampering.Police patrolled the check-in areas of the two airlines, carrying machine guns issued to them in an anti-terrorist measure imposed in 1986.

Ground staff also quizzed passengers on whether they packed their own suitcases, whether their luggage contained anything that could be used as a weapon and whether anyone had asked them to carry items for them.

Hank Auerbach, Pan Am's chief in northern Europe, said the airline had met "all the requirements" set by the Department of Transport in a directive to step up security. He refused to give details.

In France, police said security was stepped up at Charles de Gaulle and Orly airports in Paris, with special measures aimed at U.S. carriers. They declined to elaborate.

Officials at Rome's Leonardo da Vinci Airport said manpower was increased after the crash and security personnel were on alert.

The Transport Department said Wednesday that following consultations with the U.S. Federal Aviation Administration, it was "immediately directing U.S. airlines to take additional measures in Britain, in particular in relation to hold baggage."

American carriers fly to Heathrow and Gatwick airports in the London area, to Manchester in northern England and Prestwick in Scotland.

U.S. authorities have posted a $500,000 reward for information leading to prosecution of those responsible for the bombing, which killed people on board. Eleven townspeople are missing and presumed dead.

Charles Price, the U.S. ambassador to Britain, said the U.S. government was unlikely to retaliate if the killers are identified.

"I don't know of any contingency plans that involve some form of retaliation on our part and I can't imagine that it is very likely," Price said at Heathrow before boarding a British Airways plane to New York.

"I am sure that what they are primarily interested in is the same thing everyone would like to know - that is, how the bomb got there, who put it on board and where it was located."

"At this point, we don't have one particular lead or theory that we think is more promising than others," State Department spokeswoman Phyllis Oakley said in Washington. "We are simply pursuing all the leads that we have."

Foreign Secretary Sir Geoffrey Howe called on governments in the Middle East, where Moslem extremists are prime suspects in the bomb probe, to cooperate in the hunt.

The investigation is known to involve Scottish police, the FBI, Scotland Yard's anti-terrorist branch and federal police in West Germany. The New York-bound flight originated in Frankfurt with a Boeing 727.

Among the groups under suspicion are the Popular Front for the Liberation of Palestine-General Command, a Syrian-backed Palestinian faction headed by Ahmed Jibril, and the Abu Nidal group.

The Popular Front became the first terrorist organization to bomb an airliner in flight when it blew up a Tel Aviv-bound Swiss airliner in the mid-1970s.