Vietnam on Friday called on the United States, China and the Association of Southeast Asian Nations to prevent the return to power of the brutal Khmer Rouge after the planned withdrawal of Vietnamese troops from Cambodia.

Hanoi announced Thursday that it was pulling 50,000 more troops out of the country it invaded in 1978, cutting its troop strength in half as part of a plan for a total withdrawal by the end of 1990.U.S., Thai and Soviet officials welcomed the announcement as a step toward ending nearly 10 years of conflict in Cambodia, but they warned that a political solution must accompany the pullout.

"The most important issue here is what to do to guarantee the non-return of (Khmer Rouge leader) Pol Pot and the non-recurrence of the genocidal tragedy in Cambodia," an official Radio Hanoi commentary said.

The Khmer Rouge defeated the U.S.-backed regime in Cambodia in April 1975 and launched three years of radical social revolution that is believed to have killed more than 1 million Cambodians.

The commentary said China, which has steadily backed the Khmer Rouge, was scheming to take advantage of the Vietnamese troop withdrawal and restore the still dangerous guerrilla group to power.

"Therefore, public opinion demands that China, the United States and the ASEAN (Association of Southeast Asian Nations) countries assume responsibility for guaranteeing the non-return of Pol Pot," the radio said.

In Moscow, Deputy Foreign Minister Igor Roga-chev said the Soviet Union fully supports the withdrawal plan and, as in Afghanistan, "is ready to act as the guarantor of any Kampuchean (Cambodian) settlement."

Thai and Western diplomats, while welcoming the Vietnamese pullout plan, expressed concern that a withdrawal would lead to chaos and bloodshed if it were not accompanied by a political settlement.

A senior Western diplomat, who requested anonymity, said the Phnom Penh government was probably incapable of maintaining order in the country without Vietnamese military backing against the three-part resistance coalition.

The Khmer Rouge, with an estimated 35,000 troops armed by China, is the most powerful group in the coalition.