The U.S. Customs Service, intent on stopping the flow of sensitive technology to the Soviet Union, has seized and impounded indefinitely a machine called Belle, the world-champion computer chess machine.

Belle won the title at the most recent world computer chess championship tournament, in 1980 in Linz, Austria. The championships, held every three years, are sponsored by the International Federation for Information Processing and the Association for Computing Machinery.

The Commerce Department says the computer might be of military use to the Soviets. The frustrated scientist who wanted to take it to a Moscow chess exhibition--and now isn't sure he'll even get it back--has a different view: "The thing plays chess. That's all."

Customs officials said a squad of special agents spotted Belle's computer case about three weeks ago at John F. Kennedy International Airport in New York. It was marked with the logo of the Bell Laboratories, known for its work at the front edge of science. The destination stamped on the crate: Moscow.

Customs agents quickly detained the shipment and sent for instructions from Washington, according to Patrick O'Brien, chief of investigations at the Customs Service. The command post in the basement of the Customs building then turned to the Commerce Department, whose technicians and policy makers in the International Trade Administration decide whether a piece of equipment might be of use to the Soviet military.

In this case, the answer was yes.

The seizure of Belle is part of Operation Exodus, a major new program in the Customs Service to halt what government officials have called a "hemorrhage" of the nation's best technology out to the Soviet Union and its allies.

Customs officials are delighted with the new program, which they say has tripled the number of seizures of illegal exports of sensitive equipment and technology. Exodus, which has an intelligence branch, 125 new agents and a team of investigators to carry out prosecutions, has produced 1,150 leads and 370 seizures, including computers, aircraft parts and communications equipment, in its first nine months, officials say.

In a similar nine-month period last year, the Customs Service made only 122 seizures, a spokesman said.

In April, a Los Angeles company was shipping $3.5 million worth of extremely sensitive detection gear that can be used to scan the earth from a satellite. O'Brien says the scanner was bound for Moscow. Customs agents caught up with the company's eastbound plane in Houston, where the scanner was removed. Sandbags were loaded into the crate to replace the scanner and it was sent on its way with the business card of a Customs Agent attached for those who would receive the load in Moscow.

In February, two Boston companies joined together to ship $500,000 worth of electronics hardware--including a Fairchild Sentry VII integrated circuit tester--to Eastern European countries illegally, the Customs Service said. The equipment was seized in Boston and New York.

More recently, the Exodus program has been netting several shipments of equipment that might be used by the Argentine military. In one case in April, Turbine Alloy Company of Washington, N.J., was stopped from exporting jet engines that could be used in Argentine attack jets. Last week, a Hughes helicopter and three spare engines were seized on their way to Argentina.

The Commerce Department would not comment on why the chess computer could be considered militarily sensitive, but Kenneth Thompson, the scientist at Bell Labs who was responsible for the shipment, says the only way it could be used militarily would be "to drop it out of an airplane. You might kill somebody that way."

The parts of the machine said by Commerce to be sensitive, Thompson said, are items available for purchase in this country. "I just don't see the point of all this."

Even though the machine was seized two days before Thompson left for Moscow, he says customs agents failed to call him to clear the matter up or at least let him cancel the Moscow chess exhibition.

According to a spokesman at the Commerce Department, Thompson will now be subject to a penalty for violation of the Export Control Act. The possibilities range from a cash fine to losing the computer altogether.