Seven years of cautious cooperation between Vietnam and the United States to close the files on some 2,500 Americans listed as missing during the Indochina war appear seriously threatened by Vietnam's conflict with China.

Two officers from the Joint Casuality Resolution Center, the U.S. military group in charge of missing cases, were told in Hanoi last week that mobilization against China has drawn manpower and funds away from the search for graves and aircraft crash sites, Bangkok sources reported.

Popular anger over the United States' close ties with China has made many Vietnamese reluctant to assist tracing efforts, officials from Hanoi's Office for Seeking Missing Personnel were quoted as saying.

During its four-day stay in Hanoi, the U.S. team as asked to inform families of Americans missing in action that the search would continue. But in view of Washington's current attitude toward Hanoi, speedy progress is unlikely, the Vietnamese said.

China and Vietnam fought a month-long border war in the spring of 1979. Hanoi often has accused the United States, which several months earlier had normalized relations with Peking, of complicity in China's attack. Vietnam and the United States have no diplomatic ties.

A statement issued by the U.S. mission in Bangkok after the Americans left Hanoi said the Vietnamese had provided no new information on missing in acton cases and declined requests for regular meetings.

"We would hope that they will reconsider," the statement said.

Vietnamese officials did request further information on four specific cases and disclosed details of their tracing procedure. These were considered the only positive results of the meeting by the American side, Bangkok sources said.

Cooperation between Vietnam and the United States on missing Americans began in 1973, when the Paris peace accords were signed and American combat forces withdrew from Indochina. At that time, 2,600 U.S. servicemen and civilians were listed as missing or killed but not recovered.

Since then, Vietnam has returned the remains of 71 Americans and Laos the remains of two. An additional 23 sets of remains were covered by joint Center teams in South Vietnam before Communist troops captured Saigon in 1975.

Seven years after the search began, the joint center maintains a staff of 15 at its headquarters in Hawaii. Three more persons stationed in Bangkok spend much of their time interviewing refugees newly arrived from Indochina.

With little serious hope remaining that some missing Americans may still be alive, they seek details on how specific victims died or where they are buried. aPassage of time has led the military to reclassify as presumed dead all but 30 of the 2,500 unresolved cases.

Vietnamese officials argue that their government has devoted considerable time and resources to the search.Details of the exhumation of the body of one U.S. Air Force flyer, Maj. Arthur Mearns, were published in a white paper in April.

U.S. documents showed him as having disappeared over Quang Minh Province in 1966, when in fact he went down in Ha Bac Province, where local villagers hastily buried his body.

Ten years later, a Vietnamese team from Hanoi was unable to locate the site until it brought one of the men who had dug the grave from a new home 250 miles away. Six men searched with hand tools for five days but found nothing. mFinally a bulldozer was brought in and eventually uncovered the grave.

Despite efforts like these, Vietnamese officials say the United States continues to make political capital by charging that Hanoi is withholding information and remains. Hanoi has denied with unusual energy recent claims from an unidentified refugee that the remains of 400 Americans were being stored in a mortuary on Ly Nam De Street in Hanoi.