Vietnam has indicated a willingness for the first time to let U.S. technicians search for the remains of missing American servicemen at sites where U.S. aircraft crashed during the Vietnam war, a senior defense official said yesterday.

Assistant Defense Secretary Richard L. Armitage said the United States previously failed to obtain Hanoi's approval to "excavate" the several hundred places where U.S. planes went down in the 1960s and 1970s.

But last week a U.S. technical team that visited Vietnam was taken to the site of a B52 bomber crash outside Hanoi and allowed to conduct a preliminary survey to determine what equipment and manpower would be needed to dig and search, Armitage said at a Pentagon briefing.

While Vietnamese authorities have yet to approve excavations of the crash sites, he said, their willingness to permit a preliminary survey outside Hanoi indicates they "certainly are leaning in that direction."

"We assume some form of cooperation on an excavation site would be forthcoming," said Armitage, the Pentagon's chief adviser on MIA (missing in action) affairs.

The visiting U.S. team was provided the names of 26 missing American servicemen whose remains Hanoi promised to turn over to the United States, the largest such return since the war ended 10 years ago.

Armitage called the Vietnamese pledge a "positive step" possibly foreshadowing cooperation on site excavations. He said the Pentagon would "certainly look very positively" at stationing technicians in Vietnam full-time if Hanoi "could keep a technical team permanently busy."

He indicated that permission to dig up crash sites would be a significant breakthrough in efforts to account for the 2,464 Americans still missing in Indochina, including 1,820 in Vietnam. Since the end of the war, the remains of 116 Americans have been returned, including 99 from Vietnam.

He said the crash sites could contain a "good number" of airmen who went down with their planes, but he declined to estimate how many.

"We have stated our desire to excavate if they would allow us," Armitage told reporters. "If not, then we'd like to work with them to show them how we'd do it."

"We know that the Vietnamese have a lot more information than thus far they've been willing to share with us," he said. "We know that there are a lot more investigations they could be conducting and we could be conducting with them if they allow it."

The U.S. technical mission that visited Vietnam last week was making one of six annual visits allowed under an agreement worked out this year.

Hanoi officials have not indicated whether they will expand the number of crash sites available to U.S. teams but "they are entertaining these thoughts," said Armitage, who added that "things appear to be moving in the right direction."

He said U.S. experts who recently excavated the site in Laos where an AC130 gunship crashed in 1972 recovered the remains of 13 American servicemen.

Armitage said the administration has not ruled out the possibility that some Americans are still alive in Vietnam. He said U.S. officials are investigating 123 of 774 reports of such sightings since the war's end -- including one this year and four in 1984.

He said Hanoi has denied that any Americans are in captivity or at large in Vietnam.