Eastern Airlines demanded that the Customs Service release a $14 million jetliner impounded after agents found 56 pounds of cocaine aboard, saying it is an ally, not an enemy, in the war against drugs.

Monday's discovery of a bag of cocaine aboard an L-1011 a day after it arrived here from Bogota, Colombia, was the fourth drug confiscation involving an Eastern flight since August.The Customs Service said the plane would not be released until Eastern pays a fine of $896,000 - or $1,000 an ounce - and pledges to improve drug security.

Eastern executives, who are appealing $13 million in fines from the three earlier seizures, said they found the latest action difficult to understand.

"We are not an adversary. We are cooperating with the government," said Abraham Azzam, a former U.S. Drug Enforcement Administration agent hired as the airline's top drug security officer.

"We provided information about the possible shipment of drugs and we're being punished for providing the information," added airline spokeswoman Karen Ceremsak. "We don't believe this will help the good working relationship we have with the federal agencies."

Azzam said the Miami-based airline is spending $3 million on security, but drug traffickers will continue to use airliners to transport contraband.

Agents discovered the cocaine in an orange nylon U.S. Mail bag with a standard baggage tag. Flight 505 had proceeded from Miami to New York on Sunday but was seized soon after it made the return trip Monday, Customs officials said.

"We are not asking Eastern Airlines to become a law enforcement officer," said Robert Gomez, assistant district director of the Customs office at Miami International Airport. "Someone who has access to the airline has to be involved here and in Colombia.

"The government is not at war with the airlines," he added. "But it's not enough for the government to do it alone."

In the four seizures since August, 494 pounds of cocaine has been found aboard Eastern aircraft. Since March 1982, Customs has seized 20 planes, a fourth of them private aircraft, at Miami International Airport, agency spokesman Clif Stallings said.

Azzam estimated that the latest jet seizure was costing Eastern $40,000 to $50,000 a day. The carrier has trimmed back international flights as part of cost-cutting efforts, which include a plan to lay off 4,000 workers.