Southeast Asian diplomats expressed satisfaction today that their new peace proposal for Cambodia won the approval of the United States, but the main target of the plan, Vietnam, issued a strongly negative statement.

The foreign ministers of the Association of Southeast Asian Nations were informed by Secretary of State George P. Shultz this morning of his approval of the proposal for indirect talks between the anti-Vietnamese Cambodian guerrilla factions on the one hand, and Vietnam and the Vietnamese-installed Cambodian government on the other.

ASEAN made no formal response to Shultz's announcement, but several Asian diplomats said they took satisfaction from the U.S. backing, especially since Shultz earlier had been critical of the proposal. Malaysian Foreign Minister Ahmad Rithauddeen raised hopes that the proposal might bring early negotiations over Cambodia by revealing that the Vietnamese ambassador here, Nguyen Can, had called at the Foreign Ministry late yesterday to ask for details of the proposal.

Rithauddeen quoted the Vietnamese envoy as saying Hanoi had not rejected the proposal despite its harsh public attacks.

This afternoon, the Vietnamese Embassy here issued a statement saying Can had asked for an "explanation" of why ASEAN formally had endorsed a proposal that was "already rejected" by Vietnam.