Southeast Asia's noncommunist nations today warned Vietnam against allowing Indochina to become the cockpit of Chinese-Soviet conflict and appealed for Hanoi's participation in next month's international conference on the Vietnamese occupation of Camboida.

Opening a conference of the Association of Southeast Asian Nations (ASEAN), the foreign ministers of the Philippines, Thailand, Indonesia, Malaysia and Singapore held out the prospect of economic cooperation to help rebuild Vietnam's devastated economy in return for Hanoi's withdrawal of occupation forces from Cambodia.

Brushing aside the refusal this week of Vietnam and its Indochinese allies, Cambodia and Laos, to attend the U.N.-sponsored conference July 13 in New York, the ASEAN foreign ministers left the door open for a last-minute change of mind. They indicated that a follow-up conference might be held at a later dalte and that Vietnam would be given another chance to participate.

The opening of the ASEAN meeting coincided with the arrival here of Secretary of State Alexander M. Haig Jr. who will confer with the foreign ministers Friday and Saturday.

In an arrival statement at the Manila airport, Haig said his talks would focus on the dangerous activities of Vietnam with the encouragement and with the support of the Soviet Union," as well as the Cambodian situation and the upcoming U.N. conference.

A senior U.S. official said the United States would "continue to apply political, diplomatic and a full range of pressures" to end Vietnam's 2 1/2-year occupation of Cambodia. Haig declined to spell out what other pressures Washington had in mind, and the prospect of military aid to anti-Vietnamese Cambodian groups was left in doubt.

Although the Cambodian issue dominated ASEAN's opening session, the foreign ministers issued a strong condemnation of Israel's recent air raid on Iraqi nuclear installations near Baghdad. Calling the attack "unwarranted" and a "serious violation of the United Nations charter and international law," ASEAN expressed "grave concern that this dangerous and irresponsible act would escalate existing tension in the area and pose a serious threat to international peace and security."

Among other issues likely to come up in Haig's consultation with ASEAN, U.S. officals said, was the resettlement in the United Station of Indochinese refugees. The officials said Haig intends to assure the Southeast Asian national that a U.S. domestic dispute that has delayed resettlement was recently resolved and that Washington will stand by previous commitments on refugees.

Malaysian Foreign Minister Tengku Ahmad Rithauddeen told the conference that "fresh arrivals in recent months of Vietnamese illegal immigrants in alarming numbers have once again revived serious concern and worries on our part." He said the problem "has been aggravated by the reduced rate of departures" and asked resettlement countries for continued help.

All five of the ASEAN foreign ministers appealed for Vietnam's participation in next month's international conference amid expressions of concern that the conflict in Cambodia could spread.

The Philippine foreign minister and conference chairman, Gen. Carlos Romulo, said the conflict "has projected the Sino-Soviet and Sino-Vietnamese disputes into the heart of Southeast Asia's regional politics."

But the most pointed remarks came from Singapore's foreign minister, Suppiah Dhanabalan, who painted a picture of increasing economic hardship in Veitnam and growing dependence on the Soviet Union as a result of its invasion of Cambodia in December 1978. The invasion toppled the Khmer Rouge government of Pol Pot and installed a government responsive to Hanoi.

Dhanabalan said the invasion "allowed Indochina to become the cockpit of the Sino-Soviet conflict," which the ASEAN countries wanted to avoid. He said ASEAN desires "a stable and prosperous Vietnam. But we must insist that Vietnam withdraws from [Cambodia] so that opportunities do not exist for the external powers to meedle in the destinies of the region once again."

If Hanoi's troops remain in Cambodia, Dhanabalan said, "Vietnam will continue to be isolated, aid to them from the international community opposed and denied and every nationalist group resisting Vietnamese occupation will be encouraged."