WASHINGTON (AP) _ The Federal Aviation Administration, mystified at how a bomb got aboard Pan Am Flight 103, began searching for possible security lapses by U.S. airlines abroad and directed tighter screening of passengers and their baggage.

Government officials acknowledged Wednesday that a week after the Pan American World Airways jumbo jet exploded over Scotland, killing at least 270 people, there are no hard leads on who put the explosive device on the jet or where the deed was done.

Shortly after it was confirmed Wednesday by British investigators that a bomb caused the Boeing 747 to crash Dec. 21 on a flight to New York from London, the Reagan administration promised ''all available assistance'' in investigating the bombing.

U.S. officials said there was a wide range of terrorist groups that may have been responsible. But Paul Bremer, the State Department's chief of counterterrorism, told reporters it was too early to focus suspicion on a particular group.

''We shouldn't narrow our field of inquiry until we get more evidence or intelligence that leads us in a certain direction. ... There isn't any prime suspect,'' he said.

The FAA, meanwhile, said it was ''reviewing current security measures'' being taken by U.S. air carriers abroad because of the bombing and said it was preparing ''more stringent security requirements which will afford additional protection to U.S. air carriers and the traveling public.''

The FAA in a statement did not cite specific changes that are being directed at the airlines. Agency spokesman Fred Farrar said FAA security experts are still working out details of the new requirements, some of which are likely not to be made public.

Much of the focus is expected to be on tightening passengers screening, inspection of carry-on luggage and taking additional precautions to match all baggage with boarding passengers.

''In some cases it will be a matter of degree,'' Farrar said.

The FAA is responsible for establishing security requirements for U.S. airlines both in the United States and abroad, although many security measures are imposed in cooperation with the other governments involved.

While little evidence so far has been uncovered to indicate who might be responsible for the Pan Am bombing, U.S. officials said there are a number of potential suspects. They include radical Palestinians such as the pro-Syrian Popular Front for the Liberation of Palestine - General Command, and the pro- Libyan Abu Nidal group, as well as an Iranian group, the Guardians of Islamic Revolution.

Bremer said among the tasks confronting investigators was to find out how the bomb got aboard Flight 103, where it was placed and what it was made of. It's not known whether the bomb eluded security at Frankfurt, West Germany, where Flight 103 originated as a Boeing 727, or at London's Heathrow Airport, where the flight shifted to a Boeing 747 for the trip to New York.

Senior FAA officials have acknowledged that they are perplexed by the bombing because both the Frankfurt and London airports are considered to be particularly security conscious and Pan Am had undertaken special security precautions because of a tip the airline might be a target of a terrorist bombing.

According to FAA Administrator Allan McArtor these precautions by Pan Am included having all checked bags go through X-ray, having all carry-on bags checked by hand, matching all checked luggage with boarding passengers and imposing special surveillance of caterers and others who might have access to the jetliner.

The White House pledged ''all available resources'' to help the British investigation, including assistance from the FBI. FBI Director William B. Sessions said that includes interviewing relatives and associates of passengers aboard the Pan Am jet to determine if a bomb might have been placed aboard the plane to target a specific passenger.

Last week, the U.S. government offered a reward of up to $500,000 for information leading to the prosecution of any terrorist or saboteur who caused the crash.

''The incident points up the need for the international community to adopt even more stringent air security measures,'' State Department spokeswoman Phyllis Oakley said.

She said the United States had agreed with Finnish police in ruling out any connection between the crash and a Dec. 5 telephone call to the U.S. Embassy in Helsinki from a person who said a Pan Am jet would be the target of a bombing in December. That call had prompted an alert from the FAA and caused Pan Am to increase security at Frankfurt and London, officials have said.