A Vietnamese force estimated at up to 60,000 troops has launched the deepest attack yet in Vietnam's seven-month-old war with Cambodia, according to diplomatic sources here and in Bangkok.

The sources said some elements of Hanoi's border army, thought to have been reinforced recently with hardcore veterans of the conquest of South Vietnam, had penetrated 30 to 35 miles into Cambodian territory. The ground forces were reportedly supported by artillery, tanks and aircraft, including much U.S. equipment captured in 1975. Hanoi Radio, without referring to any coordinated invasion, said its forces had mauled four Cambodian battalions and one regiment to recent fighting.

Analysts said they were surprised the Vietnamese would launch such an attack during the rainy season and at a time of great tension in their relations with China, a firm supporter of Cambodia. Sources said what information they had so far on the invasion made it appear to be a counterattack to punish the Cambodians for recent raids into Vietnam.

There was no sign so far that the Vietnamese force planned to attack Phnom Penh, the Cambodian capital, perhaps another 40 or 50 miles west of the deepest point of Vietnamese penetration. An important objective of the assault, however, appeared to be the Chup rubber plantation in Cambodia's Kompong Cham Province, considered a key position for controlling the road into Phnom Penh.

The route of the invasion was reportedly centered on the area of the Parrot's Beak, the section of eastern Cambodia that juts into southern Vietnam. Sources reported particular activity near the Cambodian border town of Mimot, just north of the Parrot's Beak.

Cambodian officials have repeatedly accused Vietnam of trying to take control of their government, particularly since the simmering conflict between the two Communist neighbors flared into open warfare late in December. Cambodia said Sunday it had thwarted another alleged coup attempt organized by Vietnam and the U.S. Central Intelligence Agency, which some observers took to be a reference to officers of the former U.S. backed Phnom Penh government still operating somewhere in the country. Hanoi called the charge a "ridiculous fabrication."

There was no immediate offical word on the reported Vietnamese invasion from Phnom Penh, Hanoi or Peking. The Chinese have been conducting their own heated propaganda assault on Hanoi for its alleged persecution of ethnic Chinese.

More than 140,000 ethnic Chinese have crossed the border from Vietnam to southern China since Hanoi began to crack down on its Chinese residents who owned private businesses or who refused to accept Vietnamese citizenship.

Some analysts speculated that the Vietnamese may have tried to force the ethnic Chinese to leave the country in part to assure the loyality of its population before launching a major assault against the Chinese-backed Cambodians.

Cambodians and Vietnamese have been at odds for centuries, as the aggressive Vietnamese pushed Cambodian tribes out of what is now southern Vietnam. The latest conflict seems motivated both by Hanoi's desire for a more malleable government in Phnom Penh as part of what it sees as its natural role as leader of Indochina, and by the violent reaction of the chauvinistic Cambodians to that Vietnamese desire. Their conflict has been aggravated by the world-wide propaganda war between China, Cambodia's closest ally, and the Soviet Union, Vietnam's best friend.

After what Hanoi said were numerous assaults and atrocities commited by Cambodian troops in Vietnamese border villages, Vietnam launched a major invasion of Cambodia late last year that led to heavy fighting through mid-January. The Vietnamese forces reportedly penetrated 20-to-25 miles into Cambodia. The undermanned Cambodians, however, managed to take a heavy toll of Vietnamese troops with hit-and-run guerrilla attacks that the Vietnamese once used against the Americans.A steady supply of Chinese weapons also helped the Cambodians blunt the assault.

The Vietnamese, apparently hoping they had discouraged the Cambodians from further raids and looking for peace negotiations, then pulled back to positions on or just over the border. Skirmishes and raids continued, however, and peace talks failed to take place.

Hanoi appeared to be dissatisfied with the way its troops, including many recent recruits with little combat experience, had performed. Bangkok sources said nine crack Vietnamese Army divisions, all veterans of the war against American and South Vietnamese forces, had been added to the estimated 100,000-member Vietnamese border army. Cambodian Foreign Minister Ieng Sary told the U.N. General Assembly on June 9 that a major Vietnamese force was forming on the border and that Cambodia faced "acts of aggression and annexation on a grand scale."

An official Cambodian radio broadcast monitored here yesterday morning said the "expansionist and annexationist Vietnamese enemy has now engaged in overt aggression against our Democratic Kampuchea (Cambodia)" but said nothing specific about any current invastion from Vietnam.

Tuesday night Hanoi's official news agency reported that Vietnamese troops have reacted swiftly to several recent Cambodian border raids. During one action in Vietnam's Angiang Province, Hanoi said its forces had killed more than 100 Cambodian troops. The report said several hundred other Cambodians had been killed or captured in engagements since June 16. Analysts were skeptical that the Vietnamese troops were able to find the Cambodians - who usually rely on small guerrilla units - in such large number.