Southeast Asia's noncommunist states have endorsed an eight-point proposal to end the seven-year-old war in Cambodia between Vietnamese occupation troops and Cambodian resistance forces. The United States today expressed support for the endorsement.

The foreign ministers of the Association of Southeast Asian Nations (ASEAN) called the proposal by a three-party Cambodian resistance coalition a "reasonable" effort to "find a just and durable solution" to the Cambodian conflict. In a joint statement yesterday at an annual conference, they called on Vietnam to reconsider its rejection of the eight-point proposal, which was put forward March 17 in Peking.

A senior U.S. official today urged Vietnam to reconsider its position and "pursue real negotiations that would lead to a peaceful solution to the tragedy of Cambodia."

The official held out the prospect of western assistance for international reconstruction efforts in Cambodia and an end to Vietnamese isolation, including "normalization talks with the United States" if the Combodian conflict is settled. He said that "the question of the return to power of the Khmer Rouge does not apply under the latest proposal as, given a free choice, the Cambodian people would reject the murderous Khmer Rouge who governed from 1975 to 1978."

The Cambodian conflict was the main political issue before a conference overshadowed by the arrival of President Reagan amid a controversy over Indonesia's expulsion of three journalists, and by preparations for a Thursday meeting with Reagan and Secretary of State George P. Shultz.

The six-member association is made up of Indonesia, the Philippines, Malaysia, Thailand, Singapore and Brunei.

The final communique issued yesterday after two days of talks called the scheduled meeting with Reagan "a significant milestone in the ASEAN-U.S. dialogue." ASEAN officials said the group hopes to influence participants in the forthcoming Tokyo economic summit to resist protectionist pressures.

The foreign ministers also recommended the convening of an ASEAN summit meeting in Manila in the latter half of 1987.

In endorsing the eight-point Cambodian peace plan, ASEAN stressed that it was put forward by Cambodians and could "serve as a constructive framework for negotiation." But Indonesia indicated that it had lukewarm feelings about the plan, which Vietnam rejected immediately after it was proposed.

Indonesian Foreign Minister Mochtar Kusumaatmadja said the endorsement at this point was "meant as symbolic support" for the resistance coalition.

He said a 12-point ASEAN peace program similar to the eight-point proposal of the resistance "is still the basic framework as far as ASEAN is concerned and will be in the future."

Vietnam refuses to deal with the coalition on grounds it includes the "genocidal" Khmer Rouge. Hanoi insisted that the Heng Samrin government it supports in Phnom Penh is legitimate..

The eight-point plan calls for: negotiations with Hanoi on the two-stage withdrawal from Cambodia of an estimated 160,000 to 180,000 Vietnamese troops; a cease-fire ; U.N. recognition of the truce and withdrawal; negotiations after the first stage of the pullout between the resistance coalition and the Heng Samrin government on establishing a four-party government with Sihanouk as president and Son Sann as prime minister; free elections supervised by U.N. observers; restoration of an independent, and nonaligned Cambodia; foreign aid for reconstruction, and a nonaggression pact with Vietnam.