Southeast Asia's noncommunist nations will launch an initiative aimed at starting negotiations on a political settlement of the war in Cambodia when a six-nation conference begins Monday in the Malaysian capital, Kuala Lumpur, Asian diplomatic sources said.

The foreign ministers of the Association of Southeast Asian Nations ASEAN -- whose members are Thailand, Indonesia, Malaysia, the Philippines, Singapore and Brunei -- plan to present Vietnam with a proposal for "proximity talks," or indirect negotiations, between the warring factions in Vietnamese-occupied Cambodia, despite Hanoi's rejection of the idea, the diplomats said.

U.S. Secretary of State George P. Shultz, on the first leg of a two-week Asian and Pacific trip, Friday expressed U.S. opposition to the proposal, which is backed by former Cambodian head of state Prince Norodom Sihanouk. The plan calls for negotiations through a mediator between three Cambodian resistance groups, including the communist Khmer Rouge, on one side, and the Vietnamese and their client government in Phnom Penh on the other.

The ASEAN initiative will highlight a new round of activity on the Cambodian issue aimed essentially at demonstrating the association's flexibility and keeping the diplomatic ball in Vietnam's court, diplomats said. They acknowledged that no negotiations on the six-year-old conflict, let alone a solution, were in sight.

The two-day ASEAN foreign ministers' meeting coincides with the visit here by Shultz, who plans to fly to the Thai-Cambodian border, the scene of heavy fighting this year between Vietnamese troops and Cambodian guerrillas and frequent clashes on Thai soil between intruding Vietnamese soldiers and Thai troops.

Thai authorities see the visit by Shultz, who will be the highest ranking U.S. official to tour the border, as a welcome sign of American support for Thailand's position in opposing the presence of 160,000 to 180,000 Vietnamese occupation troops in neighboring Cambodia.

A Thai Foreign Ministry spokesman said Thai leaders would urge the United States to play a more active role in supporting the Cambodian resistance on grounds that massive Soviet aid to Hanoi is essentially underwriting the Vietnamese occupation of Cambodia. Shultz is expected to underscore U.S. commitments to Thailand, although there is no sign of U.S. desire to take the forefront on the Cambodian issue.

Symbolic of American support for Thailand, diplomats said, is a joint amphibious exercise to be launched July 12 at Thepa on the southern Thai coast. The largest of four annual joint military exercises codenamed "Cobra Gold," the mock assault on Thai beaches will involve U.S. and Thai marines and Army troops supported for the first time by U.S. Air Force F15 fighter planes.

Hanoi has denounced the maneuvers as a "crude provocation" that it said would "escalate tension . . . and strengthen the U.S. military presence in the region."

The Vietnamese also have expressed suspicion about the ASEAN proposals for "proximity talks" on Cambodia. As originally suggested by Malaysia, the idea was for the U.N.-recognized resistance coalition government of Cambodia -- composed of the Sihanouk, Son Sann and Khmer Rouge guerrilla groups -- to deal indirectly with the Vietnamese-installed Phnom Penh regime led by Heng Samrin. However, when the resistance groups objected that the plan would mean recognition of the Heng Samrin government, Thailand then proposed that the resistance groups negotiate indirectly with the Vietnamese and that any Heng Samrin negotiators be included as part of Hanoi's delegation.

Friday, Vietnamese Foreign Minister Nguyen Co Thach was quoted as saying such talks were "completely out of the question" because they were based on "a falsification of the situation" in Cambodia, where Hanoi maintains that the Heng Samrin government is in complete control.

In fact, the Vietnamese position on Cambodia appears to have hardened since a dry-season offensive from November 1984 to May this year succeeded in overrunning 20 resistance camps along the Thai-Cambodian border and driving their 225,000 Cambodian civilian inhabitants into Thailand.