Vietnamese troops are pressing ahead with their most effective dry season offensive of the six-year-old war in Cambodia in an apparent attempt to drive the remaining inhabitants of resistance settlements out of the country, according to western diplomats and military sources.

The campaign against the communist Khmer Rouge resistance group appears to be the hardest fought so far in the Vietnamese offensive, but details remain sketchy. The Vietnamese earlier overran all the major camps of the main noncommunist resistance group, the Khmer People's National Liberation Front led by Son Sann.

Thai military sources at the border have given reporters conflicting accounts of the latest fighting. According to one version, Vietnamese troops are closing in rapidly on the major Khmer Rouge stronghold in the Phnom Malai hills in western Cambodia south of the Thai border town of Aranyaprathet. Another account says the battle-hardened Khmer Rouge guerrillas, who are well supplied by their Chinese backers, have succeeded in stalling the Vietnamese drive despite heavy artillery barrages.

What is clear is that so far the Vietnamese offensive has forced about 200,000 Cambodian civilians to leave their resistance settlements and flee across the border into Thailand, western relief officials said. Now, they estimated, only about 40,000 people still inhabit resistance zones on the Cambodian side of the border.

This exodus has strained the resources of the United Nations and western relief organizations that normally provide food, water and other supplies to the Cambodian border population. It has also denied Cambodia's U.N.-recognized resistance coalition government all but a sliver of its "liberated zone" on Cambodian soil.

That was illustrated this week when the coalition's president, Prince Norodom Sihanouk, who heads the smallest resistance group in the three-party coalition, visited followers at a refugee camp about nine miles inside Thailand in Surin Province. Sihanouk plans a visit Saturday to a resistance zone inside Cambodia to greet guerrilla followers and accept the credentials of foreign ambassadors.

The planned trip to the Khmer Rouge camp of Phum Thmey, opposite the Thai border village of Ban Nong Pru, is similar to one he made there last year, but this time it appears riskier. That model Khmer Rouge village and the camps of the Phnom Malai area to the south are the targets of the Vietnamese drive, a pincers attack with thrusts from the west and south.

So far, a western military attache said, "The Vietnamese have demonstrated all along the border that they can do anything they want to."

Hanoi's apparent goals, he said, were to drive out the resistance populations, stay in place along the border to prevent infiltration by guerrillas into the Cambodian interior and discredit the resistance by denying it any permanent foothold in the country. The Vietnamese thus hope to promote acceptance of the Hanoi-installed Cambodian government in Phnom Penh under Heng Samrin as the real authority in the country.

On the other hand, the Vietnamese offensive so far has failed to destroy the resistance, leaving it largely intact on the other side of the border, western analysts said. The resistance groups hope to regain some lost ground after a few more months during the rainy season, when Vietnamese logistics become more difficult and the guerrillas traditionally have taken the initiative.

While Hanoi has been flexing the military muscles of the world's fourth-largest army lately, it also has been dropping suggestions that it wants a negotiated political settlement of the Cambodian conflict, according to diplomats in Hanoi. However, they said, Vietnamese leaders apparently hope that the military successes will enable them to start negotiations from a position of strength.

A trip to Hanoi last week by U.N. Secretary General Javier Perez de Cuellar elicited Vietnamese "clarifications" of several proposals for negotiations that are still under study by the Association of Southeast Asian Nations (ASEAN), a six-member group of noncommunist countries that strongly opposed the 1978 Vietnamese invasion of Cambodia.

However, Perez de Cuellar ended a tour of Southeast Asian countries today after conceding in Singapore that he had been unable to find "enough common ground to embark on a negotiating process" to settle the Cambodian conflict.

Foreign ministers of ASEAN -- made up of Thailand, Indonesia, Malaysia, the Philippines, Singapore and Brunei -- are scheduled to meet here next Monday and Tuesday to discuss the Vietnamese clarifications and the U.N. chief's tour.

Vietnam's diplomatic offensive of recent statements about willingness to seek a political solution in Cambodia has been dismissed by Thailand as "nothing new" and mere window dressing for old positions that essentially aim to shore up the Heng Samrin government.

At the same time, Thailand and other ASEAN members have launched their own diplomatic offensive by calling in the Soviet ambassadors to their respective countries to complain that Soviet support is helping Vietnam continue its occupation of Cambodia and endangering stability in the region.

The Soviet ambassador to Thailand, Valentin Kassatkine, responded that Moscow wants good relations with ASEAN and also favors a political solution, but that interference by a third country -- presumably China -- is a major obstacle.