The U.S. Customs Service fined Eastern Airlines $1,377,600 yesterday following the discovery by Customs officers earlier this month of nearly a ton of cocaine on two flights into Miami from Colombia.

Customs Commissioner William von Raab said the cocaine, found on two Boeing 727 jets, has an estimated street value of more than $430 million. The first shipment of 894 pounds was found on an Aug. 11 Eastern flight from Barranquilla, Colombia . The second shipment of 828 pounds was found on an Aug. 24 Eastern flight from Cali, Colombia.

The drugs were packed in unmanifested and untagged luggage hidden in the air conditioning compartments behind the forward cargo holds on both jets, according to Customs officials.

"Customs could have seized both planes, but will not because of the close cooperation between the U.S. Customs Service and Eastern Airlines in the past 18 months," von Raab said. "Eastern Airlines must by law, however, be fined the maximum of $50 per ounce of cocaine."

In a written statement released yesterday, the airline said it "is aware of the two seizures of cocaine by Customs and we regret that our aircraft have been used for drug smuggling . . . . Obviously, Eastern Airlines deplores this drug traffic and fully supports the effort of the government to reduce the traffic and apprehend those who are responsible."

Eastern said that it is temporarily suspending flights to Barranquilla "to review and adjust our security measures throughout Colombia."

The cocaine was turned over to the federal Drug Enforcement Administration, which is investigating the case. No arrests have been made.

On April 24, 1984, Customs temporarily seized an Eastern Airlines L1011 in Miami after drugs had been found on Eastern flights 22 times in the previous six months. At the time, Eastern announced new security measures and no further drugs had been found until this month.

A statement issued in Washington by Eastern Airlines President Frank Borman and von Raab said the airline and the government agency will work together to try to end drug smuggling aboard Eastern's planes.

Borman said Eastern has initiated a number of security measures and implemented many recommendations by Customs agents, but would work to further tighten security.

"We believe we have an extremely thorough program to prevent the access of controlled substances to our aircraft," the statement said.

Because of a growing tendency to try to stash drugs on commercial airlines, Customs has put special emphasis on checking airline cargoes, especially in Miami, where most drugs enter the United States.

On Aug. 7, an Air Jamaica jet was temporarily seized at John F. Kennedy International Airport in New York after 45 pounds of marijuana were found aboard.

And last February, a jet owned by Avianca Airlines, the official airline of Colombia, was seized in Miami after Customs found 2,475 pounds of cocaine concealed in a shipment of Valentine's Day flowers. The jet was returned to Avianca after the airline put up as collateral a $1 million letter of credit, a $982,400 promissory note and their foreign air carrier certificate.

Customs officials in Miami said they have seized 31,000 pounds of smuggled cocaine during the current fiscal year, which ends Sept. 30. That is nearly double the 16,691 pounds seized in the previous fiscal year.