Vietnamese troops besieging Cambodian resistance forces near the border with Thailand set fire to a major resistance settlement but are taking relatively heavy casualties as they battle guerrillas for control of it, a senior Cambodian resistance official said today.

Abdul Ghaffar Peangmith, a top official of the Khmer People's National Liberation Front, denied reports that the Vietnamese had already captured the front's Nong Chan camp, whose 22,000 civilian residents left when the Vietnamese attack began Sunday. He said that while the Vietnamese had taken part of the sprawling camp, Khmer front guerrillas were still resisting this morning in a see-saw battle with the attackers.

The assertion could not be independently confirmed, and Ghaffar conceded that the camp would not be "defended at all costs."

As the Vietnamese push continued, the Nong Chan camp's leadership allowed about 20,000 Cambodian civilians stranded just outside it to move today to an evacuation site on the Thai-Cambodian border where they can have access to relief supplies and medical care. If the situation deteriorates further, relief officials said, Thai authorities will let the refugees cross to a safer site in Thai territory.

At a news conference, Ghaffar said the Vietnamese attack on Nong Chan was "only a prelude to attacks on other bases" of Cambodian guerrillas resisting Hanoi's nearly six-year-old occupation of their country.

"I think you're going to see one of the bloodiest offensives yet," Ghaffar said.

In the past, the Vietnamese have launched annual offensives against guerrilla bases along Cambodia's western and northern borders during the November-to-May dry season, but have usually begun them around February. This year the Vietnamese appear to be starting early and are reportedly massing troops near other resistance settlements, Thai military sources said.

Ghaffar said Vietnamese troops destroyed at least one of eight sections of the Nong Chan camp, setting fire to numerous bamboo and thatch huts and larger buildings including a Red Cross clinic and the Khmer Women's Association offices.

"We shall continue to carry on our struggle in what is left of Nong Chan and recapture it if possible," Ghaffar said. "We are determined to stop the Vietnamese offensive. But if we can't stop it, we won't fight to the last man." In any case, he predicted, "Vietnamese casualties are going to rise."

So far, Ghaffar asserted, at least 120 Vietnamese have been killed in the attack, many of them by mines and mortars. He said a captured Vietnamese soldier had acknowledged that casualties were heavy and was himself found slightly wounded with bodies of 20 comrades around him.

Conflicting figures have been given for Cambodian casualties. A Red Cross official said 55 seriously wounded Cambodians from Nong Chan have been evacuated since Sunday to a hospital inside Thailand at the Khao-i-Dang refugee camp, but that the death toll was not known. Ghaffar said that only four or five Cambodian combatants were killed and 24 wounded.

He said the attackers include four Vietnamese battalions and one battalion of Cambodian troops loyal to the Hanoi-installed government in Phnom Penh under President Heng Samrin.

Ghaffar said the Khmer front was appealing to China and other unspecified countries for emergency supplies, especially ammunition for mortars, grenade launchers and recoilless rifles.