Secretary of State George P. Shultz announced U.S. support today for Cambodian negotiations proposed by noncommunist Southeast Asian nations and said that the United States would not remove its economic and diplomatic pressures against Vietnam until negotiations achieve regionally acceptable results.

In a major statement of U.S. policy prepared a foreign ministers' meeting of the Association of Southeast Asian Nations (ASEAN), Shultz assigned that organization a central role in decision-making about the six-year Vietnamese military occupation in Cambodia.

In a last-minute addition to his prepared text, Shultz gave U.S. approval to ASEAN's offer last Monday to sponsor indirect negotiations on Cambodia involving anti-Vietnamese guerrillas, Vietnam and representatives of the Vietnamese-installed Cambodian leadership.

Shultz, who earlier had been critical of the plan on the ground that it might tend to give legitimacy to what he called the "puppet" Phnom Penh rule, cited this today as a "risk" of the ASEAN proposal.

He said, however, that he has been reassured by statements about the plan made by ministers here. The proposal "deserves the backing of the international community, and the United States certainly supports it," Shultz declared.

Vietnam has been sharply critical of the ASEAN proposal, partly because it would give a negotiating role to Khmer Rouge guerrilla forces headed by Pol Pot. But the Vietnamese ambassador to Malaysia asked for clarifications of the proposal here last night and was quoted by Malaysian officials as saying that Hanoi has not rejected the proposal.

Agence France-Presse reported from Hanoi Thursday that the Communist Party newspaper Nhan Dan called the ASEAN proposal "monstrous" and a "Peking-inspired farce."

As Shultz noted today, the main thrust of the ASEAN plan is to take the diplomatic offensive by making an offer of negotiations. "ASEAN is leaving no stone unturned in the quest for a peaceful resolution of the Cambodian conflict," Shultz said.

In stating that the United States will not let up on Vietnam before a Cambodia settlement acceptable to the ASEAN countries, Shultz appeared to be providing reassurance here that Hanoi cannot obtain fundamental changes in U.S. policy simply by returning more remains of Americans missing in action from the Vietnam War.

Vietnam told a visiting U.S. technical mission a week ago that it will soon return the remains of 26 Americans, the largest such transfer since the end of the war, and seek the fullest possible accounting for all missing Americans within two years.

Shultz called this a "positive development" that would be met with unstinting U.S. efforts to resolve the MIA issue quickly.

The United States is prepared to send MIA technical teams to Vietnam on a full-time basis "if the Vietnamese are prepared to agree to a work program that would warrant such a step," Shultz said.

He added that such technical teams would in no way constitute a U.S. diplomatic presence in Hanoi and that "the normalization of U.S.-Vietnam relations is dependent upon a negotiated settlement of the Cambodian problem."

At another point, Shultz said that to obtain an end to U.S. trade and political pressures, "Vietnam will have to agree to settlement in Cambodia acceptable to ASEAN, which includes the negotiated withdrawal of its forces."

Shultz made no mention in his speech of U.S. military pressures on Vietnam. But he recited details of "greatly strengthened" U.S. naval and air forces in Asia and said that these forces "demonstrate our intention and our will to remain of paramount importance in the Pacific."

In several statements on earlier legs of his week-old Southeast Asian and Pacific tour, Shultz was critical of ASEAN's offer of indirect talks on Cambodia because of concern that it might tend to give legitimacy to the Vietnamese-backed Cambodian government in Phnom Penh.

A senior aide said Shultz mentioned this concern in a closed meeting with ASEAN foreign ministers, but then expressed the view that the final design of the proposal "had taken care of that."

Malaysian Foreign Minister Ahmad Rithauddeen, who originated the idea in somewhat different form several months ago, told reporters that he did not accept the Vietnamese comments to date as a formal rejection. He added that "we must never say die" to a political solution in Cambodia.

Shultz also conferred here with New Zealand's deputy foreign minister, Frank O'Flynn, about the ruptured security relations between the two countries following New Zealand's refusal to accept port visits from U.S. nuclear-armed warships. No progress was reported.