Vietnamese troops, pressing their current dry-season offensive in Cambodia, have overrun at least two key strongholds of the main noncommunist resistance group battling Hanoi's three-year occupation of the country.

Western diplomats and Thai military sources here believe the Vietnamese thrust against a cluster of four villages collectively known as Sokh Sann is a potentially major blow to the Khmer People's National Liberation Front, which is led by former Cambodian prime minister Son Sann.

After two days of shelling, the Vietnamese overran the front's main base, Sokh Sann 1, on March 18, a front spokeswoman said. She said Sokh Sann 2 also was captured. Thai sources have reported that the Vietnamese used tanks and heavy weapons in the assault.

There was no information on casualties at the villages, which are located in the Cardamom Mountain range in southwestern Cambodia across the border from the Thai town of Trat.

A U.S. diplomat who monitors Cambodia closely said the front's troops held off the Vietnamese for a while at Sokh Sann 3 and 4 before they were "abandoned" by the population. It was unclear whether resistance fighters still controlled the villages. The four settlements are separated by about an hour's walk through the jungle.

In recent weeks the Vietnamese also have pushed into a hilly area farther north and taken several positions from Communist Khmer Rouge guerrillas, who make up the bulk of the Cambodian resistance. For the time being, that operation seems to have stalled.

Although the front's spokeswoman sought to downplay the loss of the Sokh Sann villages, Western sources said they were important because of their well-developed facilities, including rice fields and irrigation systems. Unlike refugee camps farther north along the border, the villages were virtually self-sufficient.

Even if the Vietnamese eventually pull out, the sources said, they can deal the front a serious setback by wrecking the villages.

Diplomats and military sources said at least 3,000 villagers took refuge on the Thai side of the border after heavy shelling began March 16. The front's spokeswoman disputed this, however, asserting that only 500 civilians crossed the border out of Sokh Sann's population of 8,700.

She said the rest of the inhabitants, including fighters commanded by former Cambodian Army colonel Prom Vith, remain in the mountains.

Prom Vith is considered the front's ablest and most experienced guerrilla commander, and his force has waged the group's most effective resistance to the Vietnamese. The former officer in the Cambodian Republican Army of Lon Nol began his guerrilla campaign in 1976 against the Khmer Rouge Communists, who were then in power.

At the time, Prom Vith had only 11 men and seven guns, according to the front's spokeswoman. But he gradually built up his force and was eventually joined by a number of Khmer Rouge defectors. When Vietnamese troops invaded Cambodia and overthrew the Khmer Rouge government in January 1979, Prom Vith's guerrillas resisted the occupation.

There were conflicting accounts of the current fighting. The front's spokeswoman said "several battalions" of resistance fighters were battling at least one division of Vietnamese troops. But diplomats and military sources said they believed several hundred guerrillas were matched against two battalions of Vietnamese troops totaling about 1,000 men.

The U.S. diplomat said the Vietnamese may have used chemical weapons against the villages shortly before the main attack. He said a Cambodian official at Sokh Sann collected a sample from an alleged chemical attack March 15 and sent it to the American Embassy for analysis.

Earlier, front leader Son Sann issued a statement from his residence in Paris charging that the Vietnamese had used chemical weapons in an artillery attack on Sokh Sann March 10.

The front's spokeswoman noted that the Vietnamese also invaded Sokh Sann two years ago but that the villagers and guerrillas returned when the Vietnamese withdrew in May at the start of the rainy season.