Vietnamese forces waging their biggest dry-season offensive in six years of fighting Cambodian resistance groups virtually overran the last major guerrilla stronghold in western Cambodia today, according to Thai military and western relief officials.

The tank-led Vietnamese advance into the Phnom Malai area, long held by communist Khmer Rouge guerrillas, sent the last of the zone's nearly 40,000 civilian population fleeing into Thai territory, the officials said.

They said the Chinese-backed Khmer Rouge, considered some of the toughest guerrilla fighters in the world, were forced to cede their camps under intense artillery barrages.

Some of the guerrillas evidently gave up their weapons and joined the refugees who fled to two evacuation sites on Thai territory, witnesses said. But most split up into small units and slipped into the Cambodian interior to pursue the war with hit-and-run tactics, high-ranking military officials said. As a result of the Vietnamese offensive, relief officials said, almost all of the 250,000 Cambodian civilians who had been living in resistance settlements inside Cambodia when the fighting began in November are now in Thailand.

In the Phnom Malai area, "all of the camps are now occupied by the Vietnamese," an international relief official said. Vietnamese troops attacking the area were within hundreds of yards of the Thai border, the official said.

A western military attache said the Phnom Malai area was "basically overrun."

The commander of Thailand's eastern border task force, Maj. Gen. Sant Sriphen, told reporters near the border today that about 20,000 Vietnamese soldiers from four divisions and 20 Soviet-supplied T54 tanks were involved in the operation. He said these forces formed the two prongs of a pincer movement attacking from the east and south.

Casualties in the fighting were not known. The International Committee of the Red Cross reported having treated 23 evacuees in the past two days, of whom only eight were suffering from war wounds. The Khmer Rouge usually treat their casualties themselves.

Relief officials said about 15,000 Cambodians had gathered at an evacuation site near the Thai border village of Ban Nong Pru just across the border from the de facto Khmer Rouge capital of Phum Thmey, a model guerrilla village regularly used for special ceremonies.

Nearly 25,000 more Cambodians have fled across the border about seven miles south, the officials said.

The Vietnamese drive appears to have dealt a severe psychological blow to the Cambodian resistance in general and the Khmer Rouge in particular by erasing the last "liberated zone" of guerrillas battling the Vietnamese occupation of Cambodia.

Only Saturday, the leader of a three-party resistance coalition, Prince Norodom Sihanouk, traveled to Phum Thmey to receive the credentials of four ambassadors to his U.N.-recognized government at Phum Thmey. Khmer Rouge leaders there said the battle was going well for them and the Vietnamese were being held off and suffering heavy casualties.

While the Vietnamese offensive has displaced the resistance groups, it has apparently left them largely intact. Both Thai officials and resistance leaders remain confident that, regardless of the current Vietnamese successes, the resistance will be able to regain lost ground as it did in the past when the monsoon rains set in around May making Vietnamese logistics much more difficult.

The Vietnamese military, on the other hand, has indicated that this time it intends to remain along the Thai-Cambodian border to prevent guerrilla infiltration.