Secretary of State George P. Shultz told counterparts from a dozen nations Thursday that the United States is "unalterably opposed" to the Khmer Rouge returning to power in Cambodia.

He said the world community will continue to pressure Vietnam to withdraw all its forces from Cambodia, but added: "We look forward to the time when Vietnam will rejoin the community of nations."Beginning a nine-country trip to Asia, Shultz spoke at the opening session of a "dialogue" between the foreign ministers of the Association of Southeast Asian Nations and Japan, Australia, Canada, New Zealand, the European Community and the United States.

The three days of talks are expected to focus on the fate of Cambodia, Indochinese refugees and trade problems.

At a private meeeting among the participants, Shultz expressed cautious optimism about the results of talks starting July 25 in Jakarta between Cambodia, Vietnam, Laos and ASEAN representatives.

"We do not have an independent plan that we are proposing (on Cambodia). Our position is supportive of ASEAN," Redman told reporters.

Shultz arrived in Bangkok on Wednesday amid heightened security linked to Sunday's downing of an Iranian airliner by a U.S. warship in the Persian Gulf. Local newspapers said security personnel were doubled to about 1,000 police and soldiers.

`The United States remains unalterably opposed to the Khmer Rouge ever again taking control in Cambodia, and we believe it is essential that any settlement have adequate safeguards to prevent this," Shultz said. He noted that China, the key backer of the Khmer Rouge, recently proposed a peace plan for the war-torn nation that gave serious thought to the Khmer Rouge.

In other opening speeches, Foreign Minister Sosuke Uno of Japan and Thailand's Siddhi Savetsila called for an international peacekeeping force that would supervise free elections and prevent a new conflict in Cambodia. Uno said his country would consider funding such a force and providing personnel for it.

"By addressing the problem now, and developing effective measures to ensure that the Khmer Rouge can never come back, we also remove Hanoi's main pretext for remaining in Cambodia," Shultz said. "We must also keep diplomatic pressure on Hanoi."