Vietnam denied last night reports that it had launched a major invasion of Cambodia, while diplomatic sources here and in Bangkok said large numbers of Vietnamese troops were still in Cambodia but apparently pursuing limited aims.

Western and Thai sources said that although some forward elements of a large Vietnamese force had penetrated deeper than ever before in the seven-month-old war with Cambodia, the bulk of the invading troops were only a few miles inside the border.

Hanoi seemed to be using the troops backed by artillery and some tanks and aircraft to find and wipe out Cambodian border units which had been raiding Vietnam.

In denying its troops had invaded Cambodia in force, Hanoi radio said Vietnamese defenders had driven Cambodian raiders back across the border, "killing hundreds" and capturing many others. Many weapons and ammunition had been seized, the broadcast said.

The Hanoi broadcast said, "In recent days a big Kampuchean [Cambodian] force consisting of many regiments continuously attacked many areas in Vietnamese territory in Tayninh, Longan, Angiang and Kiengiang provinces."

The official Vietnamese news agency reported Tuesday that "the people and armed forces" of those border provinces had "put out of action" two [invading Cambodian troops] back a good distance two other battalions and one regiment.

Although Hanoi radio said its forces and "hurled (invading Cambodian troops) back a good distance, from the border line," it said reports that "70,000 to 80,000 Vietnamese troops" had penetrated 30 to 35 miles into Cambodia were "a completely groundless fabrication."

Western sources said the Vietnamese army had responded to the raids by moving in force just across the border into areas that Vietnamese troops had used as sanctuaries during their victorious war with the now defunct, U.S.-backed South Vietnamese government.

There has been much activity around the Cambodian border area town of Mimot, just north of the Parrot's Beak area where U.S. forces had also launched an invasion into Cambodia during the Vietnamese war, the sources said.

Western and Thai sources have disagreed about the number of Vietnamese troops involved in the most recent actions. Some U.S. officials in Washington had reportedly told reporters for the Voice of America and some other news organizations that as many as 80,000 Vietnamese troops may have participated in recent actions.

Diplomatic sources here and in Bangkok were largely skeptical that so many troops could actually have crossed into Cambodia, although they said Vietnamese troops still inside Vietnam may have been carrying out coordinated maneuvers in order to find and destroy Cambodian units operating on both sides of the border.

The Hanoi aim seems to be simply to clear out Cambodian forces that have been harassing Vietnamese villagers and troops along the border. The diplomatic reports were necessarily sketchy, apparently because of the need to rely on intelligence information gathered from monitoring of military radio frequencies and satellite photographs that are generally closely held.

Diplomats in Bangkok said they consider it significant that there has been no specific protest from Cambodia about the Vietnamese action. They said this suggested the Vietnamese had not attempted to establish permanent administrative bodies on Cambodian soil that would trigger Phnom Penh's deep fear of alleged Vietnamese ambitions to take over all of Inochina.

Analysts here, while noting the reports of new movements by Vietnamese border forces recently reinforced with hardcore veterans of the war against South Vietnam, said they were doubtful the Vietnamese would go very far during the rainy season and at a time of great tension on its northern border with China.

More than 140,000 Chinese residents of Vietnam, charging persecution by Hanoi, have poured across the border into southern China, further aggravating tensions caused by China's firm support for Cambodia in the border war.