Vietnam today turned over to the United States the remains of seven persons believed to be missing American servicemen.

In a related development, a team of U.S. specialists concluded the first joint excavation of a U.S. airplane crash site in Vietnam. Although the excavation about eight miles north of Hanoi proved disappointing to the American team -- the remains presented by the Vietnamese were not from that dig but from undesignated sites elsewhere -- U.S. officials expressed hope that it would open the door to more such projects.

Such a development would be in keeping with Hanoi's pledge to resolve within two years the issue of American servicemen missing in action during the Vietnam War.

These recent developments come at a time when the Vietnamese appear to be making progress on some other MIA issues as well, according to administration sources, including their willingness to discuss perhaps the most sensitive and tantalizing issue, that of "live sightings" of Americans alleged to have stayed in Vietnam after the war either as prisoners or deserters.

Vietnamese authorities say the United States wants to send a "high-level" delegation, headed by Assistant Secretary of State Paul Wolfowitz and Assistant Secretary of Defense Richard Armitage, to Hanoi Dec. 16 to pursue talks on the MIA issue.

[In Washington, State Department spokesman Bernard Kalb said there would be no meeting in December, but the possibility of a high-level visit is still under consideration.]

The United States is understood to feel a visit to Hanoi by Wolfowitz and Armitage should produce some clear progress on the MIA issue and not be portrayed by Hanoi as a move toward normalizing relations.

The just-concluded joint excavation produced only some pieces of wreckage from a U.S. B52 bomber and some small human bone fragments that specialists believe may be insufficient to yield positive identifications of crewmen.

The seven sets of remains handed over today were received by the U.S. team at Hanoi's Noi Bai airport, along with unspecified evidence on 14 other Americans unaccounted for since the war.

A U.S. C141 cargo plane flew the remains to Clark Air Base in the Philippines en route to Honolulu, where they will be analyzed at the Central Identification Laboratory.

Although the U.S. team finishing up today had reservations from the start about what they would find at the excavation site, which was chosen by the Vietnamese, they went ahead with the dig in hopes it would lead to closer cooperation in resolving the MIA issue, U.S. officials said.

The decision to allow Americans to dig on Vietnamese soil for remains represented a turnaround in Hanoi's position on the issue. As late as April, senior Vietnamese officials had told western reporters that Vietnam would not allow joint excavations until Washington agreed to establish diplomatic relations with Hanoi.

By August, however, the Vietnamese had publicly softened their position in line with a campaign to promote normalization of ties and an end to a U.S. trade embargo.

Hanoi said then that it had decided to resolve the issue of Americans listed as missing in action in Vietnam within two years, and a U.S. team that visited Hanoi in late August for talks found an unusual spirit of cooperation.

About 2,441 Americans remain unaccounted for in the Vietnam War.

Of these, 1,797 are listed as missing in Vietnam, including more than 400 who disappeared over water.

Some 556 Americans currently are listed as missing in Laos and nearly 100 others were lost in Cambodia.

Many of the missing probably will never be found. Besides the more than 400 who were lost in the waters off Vietnam, nearly 900 were missing in South Vietnam but could not be recovered while the U.S.-backed Saigon government was in power there.

Earlier this year, Laos for the first time allowed an American team to excavate jointly a crash site near Pakse, from which the remains of 13 U.S. airmen were recovered.

The United States and Laos maintain diplomatic ties, and relations reportedly have been improving.

Besides allowing the first joint excavation in Vietnam, Hanoi lately also has begun to make progress toward resolving sensitive "discrepancy cases" of Americans who were known to have been captured but whom the Vietnamese never accounted for, U.S. sources say.

Of 26 sets of remains that the Vietnamese turned over to U.S. authorities in August, 23 have been identified so far as those of American servicemen.

The 23 included three servicemen who died while they were prisoners of the Viet Cong during the war. In 1973, the Viet Cong gave the United States a list of more than 40 such cases of Americans who died in captivity, but none of their remains was returned until March this year, when two were handed over.

U.S. authorities regard it as a good sign that Vietnam is starting to resolve some of the died-in-captivity cases.

Two "discrepancy cases" among the 23 were those of Navy pilots Milton Vescelius and Randolph Ford, who were both shot down over North Vietnam.

The Vietnamese have said in each case that remains they are repatriating were recently found by search teams in the provinces.

But some Reagan administration officials believe the remains have long been accumulated in Hanoi and are being slowly parceled out.

Reagan administration officials attach credence to testimony by a former North Vietnamese mortician that he personally saw the skeletal remains of about 400 American servicemen in a warehouse in Hanoi before mid-1977.

The mortician left Vietnam in a refugee boat in 1979 and was brought to Washington for intensive questioning by Defense Intelligence Agency and Army specialists. They concluded on the basis of lie detector tests and other evidence that his information was "very credible," according to an official evaluation.

The mortician also said he saw three Caucasians in Hanoi who he was told were Americans, but was never able to identify them.

The Vietnamese continue to maintain that no American prisoners or deserters are living in Vietnam, but they have disclosed having checked out their own "live sighting" reports, thereby tacitly admitting the possibility.

While details of live sighting reports collected by Washington are classified, there does not appear to be much to support the premise of the movie "Rambo" that Vietnam still holds American prisoners of war. Nevertheless, according to recent visitors, the film's popularity is something Vietnamese authorities are painfully aware of.

According to the Defense Intelligence Agency, 788 firsthand live sighting reports have been received, mainly from Vietnamese refugees, since 1975. Of them, 83 percent have been resolved (61 percent were correlated to individuals since accounted for and 22 percent were found to be fabrications). The remaining 17 percent, amounting to 131 reports, are unresolved and under continuing investigation, the DIA says.

Of these, however, nearly 50 percent were "sightings of Caucasians who were not in a classic detention environment," meaning they were not under guard, according to Commodore Thomas A. Brooks, a DIA assistant deputy director.

He said in a speech in New Orleans in August that most of the persons in these sighting reports "could be Soviet advisers, Western European diplomats or press or other Caucasians."

If sightings of nonprisoners are deducted, "we are left with 71 reports of Americans in claimed captivity which are still unresolved and upon which we obviously concentrate our greatest efforts," he said. Brooks said about one-third of these sightings occurred before the April 1975 fall of South Vietnam.

"Only one firsthand case reporting a sighting of Americans in captivity since 1980 is unresolved," Brooks said.