The leader of the main noncommunist guerrilla group resisting the Vietnamese occupation of Cambodia today accused Vietnamese troops of slaughtering hundreds of Cambodian civilians at a refugee camp on the Thai-Cambodian border.

Son Sann, the head of the Khmer People's National Liberation Front, charged in a speech at this refugee holding center that Vietnamese troops herded Cambodian civilians into trenches and bunkers at the O Smach settlement on Cambodia's border with Thailand on Monday and killed them with hand grenades and bayonets.

It was not immediately possible to confirm the claim officially or independently.

Son Sann made the charge during a tour of the border area as fighting between Vietnamese troops and Cambodian resistance groups abated. He appealed to the United Nations and the international community to "intervene without delay to put an end to the genocide of the Cambodian people by the invading and occupying Vietnamese troops" in Cambodia.

[A Thai Air Force A37 jet fighter crashed near Kap Choen, on the Cambodian-Thai border, and military sources told United Press International that it was hit by Vietnamese ground fire. There was no official confirmation for what would be the first downing of a Thai aircraft by Vietnamese forces since a spotter plane was shot down in 1980.]

[In recent days, Thai warplanes have twice bombed Vietnamese troops that reportedly had crossed the poorly defined border.]

Officials of Son Sann's group said they were told of the alleged massacre by survivors who were brought to a hospital at Kap Choeun, in Thailand about 12 miles north of the O Smach camp, also known as Sihanoukville, which was overrun by Vietnamese forces earlier this week.

Western relief officials confirmed that the hospital holds 74 wounded Cambodians from O Smach, but they said they had not visited the camp and had no first-hand knowledge of the alleged massacre.

Son Sann said that women and children were among those "who were massacred in a cowardly way in the trenches where the [Vietnamese troops] sent them before executing them by hand grenades and finishing them off by bayonet stabs."

He said that "several hundred innocent civilians were victims of these barbarous acts" and that "mutilated survivors" could testify to the carnage.

Aides to Son Sann claimed that at least 300 Cambodians were killed in the attack, which purportedly occurred after most of the camp's population of nearly 30,000 had fled to escape the Vietnamese. The camp was defended by a few thousand fighters loyal to Cambodia's former head of state, Prince Norodom Sihanouk, who leads another noncommunist resistance faction.

According to one aide, at least one Vietnamese soldier was wounded in the incident and was brought to the Kap Choeun hospital.

After addressing supporters at this vast refugee camp, an emotional Son Sann told reporters, "I have no hate in my heart" toward the Vietnamese, "but plenty of pity for my people."

He said his forces were prepared to defend two border settlements with about 90,000 people against expected Vietnamese attacks.

But the leading military official in Son Sann's front, former Cambodian general Sak Sutsakan, said his fighters would resort to guerrilla tactics and give up territory in the face of any massive Vietnamese attack. So far about 50,000 Cambodians have fled into Thailand to escape the fighting.

Hundreds of civilians have been reported killed or wounded, many of them while trekking to refugee sites in Thailand. According to relief officials, 14 people were killed and 20 wounded in one incident April 1 when someone stepped on a mine on the way to a refugee site near here.

The Khao-i-Dang hospital currently holds 164 Cambodians who were wounded at the border, including many women and children. One woman from Cham Kakor, a settlement controlled by the Khmer Rouge guerrillas, delivered a baby in the hospital here three days after having been wounded by a shell. A hospital administrator said she was among several injured women who had given birth.

At another facility, patients could be seen hobbling painfully with new artificial legs manufactured here for mine victims.