With U.S. airlines tightening security amid a government warning about a possible hijacking plot in Europe, administration officials have assured travelers that all reasonable safeguards have been taken.

State Department and White House officials said Thursday the warning issued to U.S. carriers was aimed at upgrading security procedures rather than instilling a fear of flying in the public."Travelers on U.S. air carriers should be confident that all reasonable precautions are being taken to assure that the highest level of security exists," White House spokesman Marlin Fitzwater said.

The warning, which stems from data collected by U.S. intelligence agencies, advised security officials in Europe to be on the lookout for individuals suspected of plotting to hijack an American-owned airliner.

It was "designed to heighten the already high security procedures now in effect," Fitzwater said, stressing: "We don't want to unnecessarily scare people."

State Department spokesman Charles Redman said "the U.S. government received information about a possible attempt to hijack an American aircraft somewhere in Europe. The information was not specific as to timing, airline or flight."

"The U.S. air carriers . . . are applying enhanced security measures to counter this threat," Redman said.

NBC News said Thursday the warning was based on reports from a network of agents operating undercover in Beirut that three members of the Popular Front for the Liberation of Palestine - General Command planned a hijacking.

The network quoted an unidentified source familiar with U.S. intelligence operations as saying the undercover operatives were reliable but there was no way of knowing if the information they picked up was correct.

U.S. officials declined comment on NBC's report.

One of the first to respond to the alert sent out by the Federal Aviation Administration was Pan American World Airways, which last December lost a plane over Scotland to a terrorist bomb. The jet crashed, killing 270 people.

"We have increased vigilance to address the subject of the warning," said Pan Am spokeswoman Pamela Hanlon in New York. She said the carrier had taken appropriate action, but declined to discuss specifics.

U.S. officials dismissed a British newspaper report pinning the threat to the Easter holiday weekend but declined to discuss details on the threat. According to the London Daily Express, the memo identified three Lebanese Palestinians who may be using forged passports.

"Three Lebanese Palestinians may try to hijack a U.S. airliner in Europe," the March 17 memo was quoted as saying.

The three might be using passports from Bahrain, Pakistan, and North Yemen, which could be genuine, forged or altered, it said, adding that the information was "credible, although not confirmed."

Transportation Secretary Samuel Skinner expressed concern over the disclosure and said he directed the FAA to investigate how the warning notice had been made public.

"The major concern we have right now is to try to determine who released this information on an unauthorized basis and whether or not our sources of intellegence in the world have been compromised," he told CBS television.

The FAA sent out a security alert at Frankfurt airport two weeks before a bomb attack on a Pan Am flight that originated there, the Independent newspaper reported in London on Friday.

The Dec. 7 bulletin said that on two occasions phoney law enforcement officials questioned officials of Trans World Airlines on procedures for transporting pistols, explosives and a detonator.

The Independent said the existence of the attempts to discover how U.S. airlines guarded against bombs was disclosed by a House of Representatives' transportation subcommittee in Washington which has been holding hearings on the bombing.

It said FAA bulletin number 21 described "two incidents that may indicate attempts to test security procedures of a U.S. carrier operating from Frankfurt International Airport."