The leader of Cambodia's main anticommunist resistance group said today that his guerrillas would change tactics following the Vietnamese capture of a key resistance base on the Thai-Cambodian border and would try to operate more in the Cambodian interior to combat what he said was Hanoi's plan to colonize the country.

Son Sann, 73, the leader of the Khmer People's National Liberation Front and prime minister of a U.N.-recognized Cambodian coalition government, said the guerrillas would not immediately attempt to recapture the Ampil headquarters camp overrun by the Vietnamese Tuesday after little more than a day of fighting.

He praised the front's fighters for "accomplishing their mission" by inflicting the maximum casualties on the Vietnamese with a minimum of their own losses.

Son Sann also defended the Khmer front's decision to maintain large and vulnerable camps along the Thai-Cambodian border, and he expressed regret that his fighters did not have more arms and ammunition to defend them.

But he refused to comment on a dispute between Thai military officials and his own commanders over supplies -- particularly arms and ammunition -- that were available from a variety of outside sources, but to which the Thais reportedly blocked access during the battle at Ampil.

Now, he said, "We are going to change our tactics. We cannot fight a conventional war against the third-largest army in the world."

Son Sann made the remarks in a press conference at this Khmer front camp just across the border from a Thai village of the same name. The Vietnamese overran the camp when they opened an annual dry-season offensive against Cambodian resistance groups in November. But Khmer front forces reoccupied part of it after the Vietnamese withdrew last month. Most of the camp is currently uninhabitable because of mines left by the Vietnamese, Khmer front officials said.

They said the border, including this camp, was largely quiet today as the Vietnamese offensive appeared to go into a lull, at least where the Khmer front was concerned. There have been unconfirmed reports of fighting south of the Thai border town of Aranyaprathet between Vietnamese troops and guerrillas of the largest Cambodian resistance group, the communist Khmer Rouge.

Speaking to reporters as Cambodian civilians received U.N.-supplied rations of food and water at the western edge of this camp, Son Sann evaded questions about strains between the Thai military and the Khmer front.

He would say only that "I am very sorry we did not have on time more weapons and ammunition to inflict more casualties" on the Vietnamese attackers.

Cambodian and western officials have reported serious tensions between the Thais and the Khmer front over the resupply question, which some front leaders feel Thai authorities are manipulating to gain greater control over the guerrillas or for other motives.

Khmer front officials also expressed unhappiness about a series of meetings between Thai and Vietnamese officers at the western entrance to the Ampil camp on the Thai-Cambodian border opposite the Thai village of Ban Sangae. The meetings culminated yesterday in what a Thai commander said was Vietnamese agreement to pull back about 500 yards from a Thai antitank ditch and stay east of markers placed by Thai soldiers to delineate the border abutting the Ampil camp.

The tensions between the defeated Cambodian anticommunist guerrillas and Thai forces burst into the open yesterday at a refugee evacuation site inside Thailand. Refugees from the overrun Ampil camp panicked when Thai soldiers fired their rifles into the air during a dispute between Cambodian Gen. Dien Del, the military leader of the Khmer Peoples National Liberation Front guerrilla group, and a local Thai commander.

The dispute was ostensibly over the conduct of Khmer front soldiers from Ampil who sought refuge at another refugee settlement further north.

The Thai commander reportedly complained to Dien Del that his guerrillas had molested Cambodian inhabitants of the Dong Rek camp and threatened some of the 4,300 Vietnamese refugees who have gathered there since fleeing their homeland across Cambodia.

But underlying the tensions, according to western and Cambodian sources, is bad blood between the Khmer front and the Thai military over supplies of arms and ammunition to the defenders of Ampil and other Khmer front camps overrun by the Vietnamese.

"If we do not get resupplied, the battle is over," said a senior Khmer front official. "The lack of ammunition is putting us to sleep." He added, "I think if we had enough ammunition, we could liberate Ampil."

He said that counterattacks to try to recapture the key base, site of the Khmer front's military headquarters, were impossible without fresh supplies, which he claimed the Thais were holding back.

Asked why, he rubbed thumb and forefinger together in the universal gesture meaning money. He declined to elaborate, but noted that the Thai military controls deliveries of weapons to the guerrillas. Most of the arms come from China, but Singapore also has contributed weapons to the noncommunist guerrillas, according to diplomatic sources.

"If we expose this secret, it may close the pipeline," the Khmer front official said. "And there is no other way to get arms, so we must suffer."

A western official who monitors the Thai-Cambodian border confirmed that a shortfall of ammunition contributed to the quick Khmer front defeat at Ampil, and he said Dien Del was angry about not receiving new supplies.

The senior Khmer front official said the group's guerrillas were anxious over the situation, but he insisted that morale was still good and that the front's organization remains intact.

"We have not yet lost our spirit of resistance," he said. "The Vietnamese can destroy all our camps, but they can't destroy our hearts and our spirit. The loss of Ampil doesn't mean the loss of our morale, the loss of our resistance."

However, western officials said the behavior problems of the guerrillas at Dong Rek, the only major Khmer front camp still not overrun by the Vietnamese, indicated that morale was sinking following the Ampil setback.

While virtually no one expected the Khmer front's fledgling guerrilla organization to withstand an all-out Vietnamese attack, the apparent ease with which the Vietnamese captured Ampil evidently has dealt the front a severe blow. At the heart of the problem is a contradiction between the front's guerrilla strategy and its need to fight conventional battles to defend settlements in Cambodian "liberated" territory and thereby preserve its status as part of a legitimate government.

"We are told we must be guerrillas and we must not make conventional counterattacks," said a senior Khmer front official. "But our troops want to show their capabilities in battle. They want to show what they can do. They only need more munitions to fight the Vietnamese."

In his press conference today, Son Sann acknowledged that losing Ampil hurt the Khmer front, but he said the loss could be overcome.

"Ampil was our headquarters," he said. "But our headquarters may be anywhere now." He said the Khmer front would reorganize its forces into smaller groups and concentrate on penetrating into the Cambodian interior. But he did not explain how the front would achieve this in view of the current offensive, which appears aimed precisely at preventing future guerrilla infiltration.