Cambodia and Vietnam said yesterday that there have been thousands of casualties in the large-scale warfare that Cambodia said has spread 20 miles inside its territory.

In a new outpouring of brutal accusations and cross-claims by the two Communist nations, Cambodia said "we have killed or wounded 29,200 of the enemy" and "five divisions of the enemy have been routed."

Vietnam counter-claimed that in the long unpublicized fighting, Cambodian troops crossing into its territory "butchered" or wounded "thousands of inhabitants" while "tens of thousands of civilians have had to move farther from the border for security."

Western specialists are extremely skeptical about the military claims on both sides, since Cambodian made public its version of the war Saturday and severed diplomatic relations with Vietnam. A furious propaganda war is now under way between the two nations.

But what is not doubted by outside analysts is that there has been great bloodshed on both sides. American intelligence data confirms that there has been intensive fighting, especially since last September, along the 700-mile Cambodian-Vietnamese border.

Cambodia claimed yesterday that the fighting is continuing this week at division-sized levels and it ridiculed Hanoi's contention that the clashes are only a border dispute. Phnom Penh Radio charged that Vietnam is engaged in a "systematic aggressive offensive" with "the strategic aim of taking Cambodia's territory as Vietnam's satellite in the Indochinese federation."

Cambodian insinuations that Russian combat advisers are supporting the Vietnamese troops, became somewhat more specific yesterday.

"On the Krek battlefront," Cambodia charged, "two corps of European nationals were seen on a tank that we had fired at. They had pointed noses and red hair and wore white work clothing like iron smelting workers."

"On this front," the Cambodian report continued, "we also heard communications in Russian voices and Vietnamese nationals speaking in Russian."

The Soviet Union is the major supporter of Vietnam, while China is the foreign nation closest to the Cambodian Communists who gained power when the United States withdrew from Indochina in 1975. American specialists are very dubious that Soviet combat advisers are involved in the present fighting, but Soviet-Chinese rivalry is enmeshed in the conflict, with the Russians taking their own propaganda shots at the Chinese.

A Soviet correspondent reporting from Hanoi yesterday invoked reports "by foreign correspondents" who he said "point to the participation of numerous Chinese military advisers in military operations on the Cambodian side," the Soviet correspondent added, "At the same time Hanoi is resolutely denying the fairy tales about 'foreign advisers' taking part in aggression against Cambodia."

The prevailing view among American and other Western specialists is that Vietnam with a population of 47 million people and the great economic burdens of "digesting" South Vietnam which North Vietnam conquered in 1975, has no desire to take over impoverished Cambodia, with an estimated population of about 7 million.

What Vietnam intends, these specialists believe, is "to teach Cambodia a lesson" and probably to retain a portion of the territory it has seized. Hanois troops now dominate Cambodia's Parrot's Beak salient, which protrudes into Vietnam. This was the region included in the American-South Vietnamese drive into Cambodia in 1970, in the name of eliminating Vietnamese Communist "sanctuaries" inside Cambodia.

Vietnam, with the lengthy expertise in propaganda warfare, yesterday turned on a major public relations operation in Hanoi to respond to the original charges by Cambodia last weekend.

In a more-in-sorrow-than-in-anger tone, Hanoi's leadership produced a press conference, film clips, a "white paper" and exhibits, designed to convince a pres and diplomatic audience that it was the victim, not the instigator, of the warfare.

The pictures, it was reported from Hanoi, "showed disemboweled women, decapitated children, the bodies of adults thrown into ponds and rivers, burned out villages, churches or Buddhist temples destroyed by rockets, and tons of rice and dozens of cattle that had been destroyed by fire."

Vietnam said it repeatedly has offered to negotiate the border dispute with Cambodia which involves "complex issues left over by history," dating from the French occupation of Vietnam, Laos and Cambodia. Instead, Vietnam claimed, from the time the present rullers of Cambodia gained control, in 1975, it has engaged in "systematic encroachments" on Vietnamese territory, ultimately extending "along the entire border . . ."

Since September, Vietnam contends, Cambodia has sent "about four divisions" against Vietnam, some penetrating "10 kilometers [6 miles] inside Vietnamese territory."

Cambodian troops, Vietnam charged, engaged in "savage massacres" of Vietnamese civilians, using weapons up to 105-mm. artillery. Vietnam, giving no specifics at all on its own military actions in Cambodia, said only that it was "compelled to take self-defense actions," and is anxious to negotiate, but that Cambodia refuses to do so.

The Cambodian version yesterday made extravagant claims of military victory.They were received with total skepticism by American and other outside experts, in part because Cambodia is vastly outgunned by the hugely superior Vietnamese foreces, and in part because the Cambodian account itself acknowledged failure to dislodge the intruding forces.