Cambodian guerrillas engaged Vietnamese forces today in the fiercest fighting of Vietnam's current dry-season offensive in a bid to recapture a resistance settlement just across the border from this Thai village.

According to Cambodian resistance and Thai military sources, the fighting centered on the Vietnamese-held military headquarters of the Rithisen camp, where anticommunist guerrillas have knocked out at least one Vietnamese T54 tank that both sides want to salvage.

Casualties were reported heavy by the standards of the six-year-old Cambodian confict. Cambodian resistance sources said that at least 30 guerrillas were killed in today's attempt to recapture the camp from the Vietnamese, and Red Cross doctors reported treating more than 90 Cambodian wounded. Vietnamese casualties were unknown.

It appeared to be the costliest operation for the anticommunist Khmer People's National Liberation Front, which had controlled the camp, since the guerrilla organization was formed in 1979 to resist the Vietnamese invasion of Cambodia.

Pairs of Cambodian guerrillas trudged along a dusty road today past a Thai military checkpoint on the border opposite the Rithisen camp, carrying out the wounded on litters as Vietnamese artillery boomed behind them on the Cambodian side of the border.

The heaviest fighting was reported early this morning, when Vietnamese artillery pounded Cambodian territory for about six hours in an effort to repulse a guerrilla attack. But the wounded still were being carried out, and sporadic shelling and small-arms fire still could be heard late this afternoon.

The seriously wounded were loaded into Thai pickup trucks or Red Cross ambulances to be taken to the Khao-i-Dang refugee camp in Thailand, where a hospital run by western relief agencies was crowded with surgical patients.

Vietnamese troops, waging a seasonal offensive against resistance groups along the Thai-Cambodian border, overran most of the Rithisen camp Dec. 25, forcing its 62,000 civilian residents to flee to an evacuation site across the border.

Aided by reinforcements, the guerrillas later counterattacked the approximately 1,000 Vietnamese troops holding the camp and recaptured three-fourths of it, according to Hing Kunthon, a member of the Khmer front's executive committee and deputy chief of its general staff.

In an interview today at the front's Ampil headquarters camp in western Cambodia about 18 miles north of Nong Samet, he said guerrilla morale had been lifted by the knocking out of two Vietnamese T54 tanks at Rithisen.

Hing Kunthon said that one tank, blown up by a land mine east of the Rithisen camp, already had been hauled away by the Vietnamese but that both sides were battling to take control of a second one hit by rocket-propelled grenades Dec. 28 in the camp.

The Thai deputy commander of the Eastern Border Task Force, Col. Chettha Tharnatho, could confirm only the disabling of the tank hit on Dec. 28 and estimated that the guerrillas had recaptured about half the camp. He said the camp, which the Vietnamese never had attacked before, was weakly defended by ill-prepared Khmer front troops.

Hing Kunthon conceded that the front's forces at Rithisen were inexperienced. But he said the guerrillas there, now numbering more than 3,000, were prepared to fight back despite their light armament.

The battle for the camp is seen shaping up not only as a demonstration of the Khmer front's capabilities in combat, but as a test of Vietnamese willingness to hold on to territory it has gained this dry season in driving Cambodian refugees across the Thai border. About 100,000 of the border area's 250,000 Cambodian population have fled to the Thai side since the Vietnamese onslaught began in November.

The brunt of the offensive so far has been borne by the Khmer front, but fighting also has been reported between the Vietnamese and two other resistance groups, the communist Khmer Rouge and a small faction loyal to Prince Norodom Sihanouk. The Khmer front, led by Son Sann, is in an uneasy coalition with the noncommunist Sihanouk group and the Khmer Rouge, which fields the largest military force with about 30,000 to 40,000 guerrillas.