A joint U.S.-Laotian search team excavating the site of a 1972 American military plane crash in southern Laos has found remains believed to be those of missing U.S. servicemen, according to an American military official here.

The discovery was made soon after the excavation work began Monday, the official said. But U.S. authorities will not know how many sets of remains there are and to whom they belong until they are taken to Honolulu for identification after completion of the work Feb. 21, he said.

Thirteen of the 16 crewmen aboard the AC130 gunship have been unaccounted for since it crashed in rugged terrain about 24 miles northeast of Pakse in southern Laos on Dec. 21, 1972. Two crewmen parachuted to safety, and the remains of another were picked up the next day by a rescue helicopter.

The excavation of the Pakse site represents the first time that U.S. specialists have been allowed to search for remains of Americans missing in action during the Vietnam War since it ended in 1975.

U.S. officials hope that the Communist governments of Laos and Vietnam will allow more such projects, but Hanoi has made it clear that it wants U.S. agreement to open diplomatic relations as a prerequisite.

Although Laotian authorities previously have taken visitors to see the Pakse crash site, it has remained "relatively undisturbed" and the excavation work has been going well, said the U.S. military official, who did not want to be identified.

Other American officials confirmed that the project was off to a good start, but declined to discuss what the searchers had found at the site.

The plane was flying a mission over the Ho Chi Minh Trail, the North Vietnamese infiltration route into South Vietnam, when it was hit by ground fire that caused fuel to pour into the cargo section, according to the official military account of the crash.

A midair explosion then blew the two surviving crewmen out the plane's open ramp door, and the gunship crashed in sparsely populated jungle in the area of the Bolovens Plateau.

The 13 crewmen are among 576 Americans listed as missing in Laos since the Vietnam War. A total of 2,483 Americans, including 42 civilians, are still unaccounted for in Vietnam, Cambodia and Laos. All but one, an Air Force pilot who went down in Laos in 1965, are classified as presumed dead.

The U.S. team in Laos consists of 10 military personnel and two civilians, including a representative of the U.S. Embassy in Vientiane, the Laotian capital, officials here said.

They said the military contingent includes specialists from the Joint Casualty Resolution Center and Central Identification Laboratory in Hawaii and two members of an explosive ordnance disposal unit.

American officials have been waiting to conduct the excavation since December 1983, when a U.S. team visited the site to do a preliminary survey.

The same site had been visited in 1982 by members of the National League of Families, a private group dedicated to accounting for prisoners and other U.S. servicemen missing during the Vietnam War. The delegation included Ann Hart of Pensacola, Fla., whose husband, Air Force Maj. Thomas Hart, is one of those unaccounted for.