Cambodia's largest guerrilla group, the communist Khmer Rouge, Friday became the last rebel force to agree to a temporary truce beginning May 1, in the 12-year-old war against the Vietnamese-installed government.

The proposal had been accepted by Vietnam, the Vietnamese-installed government and the two non-communist groups allied with the Khmer Rouge - the forces of Prince Norodom Sihanouk and the Khmer People's National Liberation Front."We accept the call . . . and we hope Vietnam will abide by the call," Khmer Rouge leader Khieu Samphan said after he and leaders of the two other rebel groups met in Bangkok Friday with Thai Prime Minister Anand Panyarachun.

The truce was jointly proposed on Monday by France, Indonesia and the U.N. secretary general, the main brokers in the Cambodian peace process. They said fighting should be halted from May 1, at least until the completion of peace talks expected sometime in May in Jakarta, Indonesia.

The truce is designed to facilitate the talks, which will focus on the Vietnamese-installed government's objections to a U.N. Security Council peace plan. The guerrillas have fully accepted the plan, under which the United Nations would be extensively involved in the country's civil and military affairs in the period before elections.

In further efforts to ease the way for the talks, Anand met with the guerrilla leaders Friday and will meet Sunday in Bangkok with Hun Sen, prime minister of the Cambodian government. Hun Sen currently is in Tokyo for medical tests.

Hun Sen's government has insisted, among other demands, that the U.N. plan include specific measures to prevent the return to power of the Khmer Rouge.

The Khmer Rouge massacred hundreds of thousands of people with radical agrarian policies when it ruled Cambodia from 1975 until Vietnam invaded in late 1978 and installed another communist government.

Fighting has died down with the start of the rainy season in April.

During the dry season, government forces backed by helicopter gunships tried to retake positions in the west lost since a major Vietnamese troop withdrawal from Cambodia in September 1989. The guerrillas used artillery and tanks for the first time.