The Vietnamese government has agreed for the first time to let U.S. technicians search for the remains of missing American servicemen at a site where a U.S. bomber crashed during the Vietnam war, a Defense Department official said yesterday.

The agreement is considered a significant step in U.S. efforts to gain access to hundreds of crash sites that are thought to be the largest single concentration of the remains of missing servicemen. A total of 1,802 are listed as missing in action in Vietnam.

Vietnamese officials, meeting with U.S. representatives late last month in Hanoi, announced their decision to permit a "joint excavation" of a site east of Hanoi where a B52 bomber crashed in December 1972, said Commodore James D. Cossey.

Hanoi's representatives also responded for the first time to specific refugee reports of living Americans in Vietnam, Cossey said. Previously, Hanoi had issued blanket denials when asked about such "live sightings." In September, however, officials claimed that they investigated reports of three sightings and found them untrue, he said.

Washington demands a withdrawal of Vietnamese troops from Cambodia as the prerequisite for normalizing relations with Hanoi. Cossey said while the issue of missing U.S. servicemen is "unrelated" to normalization, Hanoi's help may "lay a foundation" for reconciliation.

Cossey said the first crash-site exploration in Vietnam will take place in the "near future" after U.S. and Vietnamese officials work out technical details.

He said the United States will provide supplies, equipment and military engineers and technicians to work alongside Vietnamese in digging up the site and searching for remains of servicemen. Expenses will be borne by the United States, he said.

"We are hopeful that this excavation, if it can proceed successfully, will be the first of many to occur in Vietnam," said Cossey, acting deputy assistant defense secretary for East Asian and Pacific affairs.

He said he did not know which bomber crashed at the site to be excavated or how many men were aboard. But he estimated that the remains of about 500 U.S. servicemen are buried in crash sites in Vietnam.

An 11-member U.S. team conducted the first excavation in Vietnam in February, recovering the remains of 13 servicemen at a site in Pakse, Laos, where an AC130 gunship crashed in 1972.

A total of 2,446 American servicemen are still reported missing in Vietnam, Laos, Cambodia and China 10 years after the war ended.

U.S. officials have been seeking permission to dig up crash sites in Vietnam since "technical talks" on missing U.S. troops began in December 1982.

At talks in July, the U.S. team was provided the names of 26 missing servicemen and allowed to conduct a preliminary excavation survey at a B52 crash site outside Hanoi.

Formal permission for the excavation came in the ninth session of talks, Sept. 25 to Sept. 29, which the Pentagon described as the "most open and cordial" to date.

At the same session, Cossey said, Vietnam denied three reports of "live sightings" of Americans, including one of a black man. Authorities claimed he is a "black Khmer," one of Southeast Asia's ethnic groups, Cossey said. He said the U.S. team considered that "very unusual" and will raise the issue at the next meeting.

But, he said, Vietnamese willingness to discuss specific cases of sightings is "a very hopeful sign." At the same time, he said, U.S. officials have "no evidence" of Americans living in Vietnam.

Almost 800 reports of living Americans have streamed in since the fall of Saigon in 1975. Cossey said U.S. intelligence tries to determine their accuracy but that "ultimately, the resolution of this issue depends mostly on cooperation from the Indochinese governments."