After a day of intense shelling, Vietnamese forces spearheaded by tanks and armored vehicles today thrust ahead with a major ground assault against the main stronghold of Cambodia's communist Khmer Rouge guerrillas in rugged, hilly terrain near the border with Thailand, Thai military officials said.

The Vietnamese assault reportedly was met by heavy resistance from Khmer Rouge guerrillas, who have been battling the Vietnamese since being driven from power in Cambodia in January 1979.

Results of the fighting were not immediately clear, but it appeared from Thai military reports that Vietnamese units had penetrated the Phnom Malai area of Khmer Rouge bases and were driving toward the guerrillas' military headquarters at Hill 329, about 3 1/2 miles southeast of the Thai border town of Ban Nong Pru. The Phnom Malai area also features a string of Khmer Rouge settlements along the border, including the model village of Phum Thmey, the organization's de facto capital, opposite Ban Nong Pru.

Western relief officials said most of the 36,000 civilians in the Khmer Rouge camps had fled across the border into Thailand to escape the fighting. Unconfirmed reports from the border said 45 Cambodian civilians had been killed and 130 wounded in the Vietnamese shelling.

Thousands of Thai villagers also have fled the border area, and three were reported killed by shells that overshot the Khmer Rouge camps.

A Red Cross official said that, in a departure from their usual strict policy, Khmer Rouge authorities had allowed the Red Cross to evacuate seven Khmer Rouge casualties this morning for treatment in Thailand.

Besides the battle for Phnom Malai, the Vietnamese have been attacking Khmer Rouge bases to the south opposite Thailand's Trat Province, forcing about 10,500 Cambodians to flee across the border, relief officials said.

The latest round of fighting apparently also has increased tension on the border between Vietnam and China, which backs the Khmer Rouge.

Radio Hanoi today said that Vietnamese forces have "put out of action" 250 Chinese troops who allegedly crossed into Vietnam. The radio also accused China of firing thousands of artillery rounds into four northern Vietnamese provinces since Monday.

Nevertheless, western diplomatic sources said they expected the Vietnamese to go all out to capture the Phnom Malai area, which has eluded their grasp for six years. The preparations Vietnam has made for the current offensive and the forces committed to it -- said to include elements of four infantry divisions -- exceed those of any Vietnamese operation against the Khmer Rouge in recent years, the diplomats said.

"The Vietnamese are not going to come this far without taking the terrain," a Western military attache said.

Besides its value as a major Khmer Rouge stronghold, the Phnom Malai area has symbolic importance for the resistance group. The main entrance to it from the Thai border is the showpiece settlement of Phum Thmey.