Secretary of State George P. Shultz today charged that Vietnam was deliberately withholding the remains of several hundred American servicemen killed during the Vietnam War.

Shultz made the charge at the annual conference of the foreign ministers of Southeast Asia's noncommunist states and several western countries. He urged Vietnam and its Communist neighbor, Laos, to cooperate in resolving the issue of Americans missing in action in Indochina and appealed for assistance from the Association of Southeast Asian Nations (ASEAN), which sponsored the conference.

Shultz's comments were believed to mark the first time that the charge has been leveled by a ranking U.S. government official. The accusation has been made repeatedly in congressional testimony by relatives of missing servicemen and MIA specialists.

The ASEAN meeting concluded today with a pledge by the United States to maintain strong support for the group's opposition to the Vietnamese occupation of Cambodia and to continue taking in Indochinese refugees who flee to the ASEAN countries.

At a closed session with the ASEAN foreign ministers representing Thailand, Indonesia, Malaysia, the Philippines and Singapore. Shultz said Washington would welcome information on the MIA issue.

According to a U.S. spokesman, Shultz told the ministers, "We believe several hundred remains are being deliberately held by Vietnam." Shultz called this "a cruel and heartless action" on Hanoi's part.

The U.S. spokesman suggested that the remains were being withheld as a "bargaining chip" for improved ties with the United States.

Shultz later repeated the charge at a joint news conference, but he declined to speculate about the Vietnamese motive for allegedly keeping the remains.

Hanoi has repeatedly denied withholding any American remains or captured servicemen.

In contrast to the public denunciations of Vietnam over the MIA issue and the occupation of Cambodia, the U.S. ambassador here, John Gunther Dean, has been holding a series of unpublicized meetings with the Vietnamese ambassador to Thailand to discuss matters of common interest, diplomatic sources said. Some of those meetings have concerned the MIA issue and arrangements for visits to Hanoi by teams of U.S. specialists, the sources said.

The meetings sometimes have been called by the Vietnamese to inform Washington in advance of Hanoi's public statements on such matters as partial withdrawals of Vietnamese troops from Cambodia. At least one this spring was initiated by the United States to express concern about Vietnamese incursions across the Cambodian border into Thailand.

According to U.S. sources, the United States has stressed to Hanoi that the talks are not meant to promote political normalization, which Washington has spurned because of the Cambodian occupation.

It was not immediately clear how Shultz's charge would affect these talks or a series of regular visits to Hanoi by American military delegations to discuss MIA matters.

During the latest visit early this month, the Vietnamese turned over nine boxes said to contain the remains of American servicemen, plus identification cards for three others. Vietnamese officials indicated at the time that the remains had been recovered since the last trip by the U.S. team in March. The remains were flown to Hawaii for positive identification at a military laboratory, which has yet to release its findings.

According to U.S. sources, Shultz's allegation today stems from congressional testimony by a Vietnamese refugee who said he had worked as a mortician in Hanoi and had helped process the remains of 400 American servicemen at a storage facility in the Vietnamese capital.

The Vietnamese have since shown the compound to visiting American journalists, who saw nothing there. However, the Defense Intelligence Agency has attached credence to the refugee's testimony and believes the remains may now be kept somewhere else.

A U.S. spokesman said 2,494 American servicemen were still unaccounted for in Indochina from the Vietnam War era. All but two now are officially listed as presumed dead.

Asked about the issue at today's joint news conference, Shultz said, "We have intelligence that suggests the remains of quite a sizable number of U.S. servicemen are in hand but have not been turned over to us."

In response to other questions about private forays to look for American POWs or MIAs in Indochina, Shultz would not rule out administration support for such ventures if they stood "a good chance of being effective." However, U.S. officials traveling with Shultz later said he did not mean to condone private armed incursions into Indochinese countries but was referring to efforts by MIA relatives and citizens' groups.