Facing the most aggressive Vietnamese offensive along the Thai-Cambodian border since 1979 and unable to defend its large settlements, Cambodia's main anticommunist resistance group appears to be reaching a turning point in its struggle against Hanoi's six-year-old occupation of Cambodia.

Since mid-November, the Khmer People's National Liberation Front, led by Son Sann, has lost six camps along the border to Vietnam's dry-season onslaught, and another has been largely evacuated in expectation of an imminent Vietnamese attack.

The latest to fall was the Ampil camp, a showcase settlement of 23,000 Cambodians about 40 miles northeast of the Thai border town of Aranyaprathet. The camp also was the site of the front's military headquarters.

The loss of Ampil showed that while the front has made great progress since 1979, when it started with virtually nothing, its approximately 15,000 fighters are still no match for the Vietnamese and remain militarily a junior partner in a resistance coalition with the widely hated but powerful Khmer Rouge communists.

The defeat also underscored a necessity for the Khmer front to adapt to guerrilla tactics and try to avoid being pinned down on the border by superior Vietnamese forces drawn from the estimated 160,000 to 180,000 occupation troops in Cambodia.

But the front evidently faces a major and daunting task in attempting to push its struggle into the Cambodian interior with guerrilla methods.

Guerrilla struggle is largely alien to the front's military leadership, made up mostly of officers who served under the Lon Nol government that took power in a 1970 coup and was toppled by the Khmer Rouge in 1975. They and the camp leaders who have emerged as local warlords seem to value their settled life styles in the resistance bases, where they acquired -- considering the circumstances -- relatively comfortable homes with such amenities as video players and gardens.

There they were also saddled with the defense of large, concentrated populations of refugees, totaling more than 150,000 of the estimated 250,000 Cambodians who live along the border under the control of the three anti-Vietnamese resistance groups.

What to do with these 150,000 people, who are both a strength and a liability for the Khmer front, remains a major problem. Most are now encamped on the Thai side of the border after having been evacuated from their settlements in the face of the Vietnamese offensive.

Other difficulties impeding the front's development as a guerrilla force in Cambodia include shortages of arms and ammunition and intensified Vietnamese efforts to stop guerrilla infiltration. The efforts include the construction of a defensive barrier inside Cambodia parallel to the border, for which the Vietnamese reportedly have mobilized thousands of Cambodian civilians.

A major dilemma for the front is that, although it enjoys western diplomatic support and lends acceptability to the three-party coalition, it lacks a committed arms donor willing to keep it well supplied. Most of the front's weapons have come from China, but Peking always has shown more generosity to the communist Khmer Rouge, whose 30,000 to 40,000 guerrillas pose the greatest military threat to the Vietnamese.

The third party in the resistance coalition, a faction loyal to former chief of state Prince Norodom Sihanouk, is a relatively negligible military force with 5,000 to 8,000 fighters and has not yet come under attack in the current Vietnamese offensive.

In fact, there have been indications that Hanoi would like a separate deal with Sihanouk that would give credibility to the Vietnamese-installed Heng Samrin government in Phnom Penh and undermine the resistance coalition. When Sihanouk was visiting Paris recently, according to Cambodian and western sources, the Phnom Penh government's foreign minister, Hun Sen, attempted to see him, but the prince refused a meeting.

The Khmer front, apparently the primary target in the current offensive, is bearing the brunt of a Vietnamese strategy evidently aimed at discrediting the organization as a political threat to Hanoi's proteges in Phnom Penh and buying time for the government to consolidate its hold. To counter the Vietnamese strategy, western diplomats say, the Khmer front must be able to operate in Cambodia and raise the cost of Vietnam's occupation of the country. Combined with the international isolation and the resultant economic costs brought on by Vietnam's invasion of Cambodia in December 1978, the diplomats say, increased military hardship there might induce Hanoi eventually to negotiate a political settlement.

In the short term, western diplomats say, much depends on how Thailand reacts to the Khmer front's losses.

Front officials, including Son Sann, have complained that the group's guerrillas were hampered in defending Ampil and other camps by shortages of arms and ammunition, deliveries of which are controlled by the Thais.

Western diplomats say Thai authorities may have perceived a defense of Ampil as hopeless and wished to cut guerrilla losses. While the guerrillas managed to hold off a Vietnamese drive against the camp last April, this time the Vietnamese committed three or four times more troops, tanks and artillery to the battle and apparently overcame previous logistics problems.

As it happened, according to Son Sann, the Ampil defenders suffered only six killed and 83 wounded, 20 seriously, and managed to pull out of the camp with their weaponry largely intact.

In a press conference on the border Friday, the 73-year-old former prime minister under Sihanouk also said that his guerrillas destroyed eight tanks and armored vehicles and damaged six others before withdrawing in the face of four Vietnamese regiments totaling up to 4,000 infantrymen backed by heavy artillery.

Another factor in the Vietnamese success was a short and relatively mild rainy season in which the guerrillas were unable to harass the Vietnamese forces in the interior as they had previously.

The current offensive began earlier than expected in November, and the Vietnamese have plenty of time left to strike at the resistance groups until the dry season ends around May.