The Soviet Union charged presidential national security affairs adviser Zbigniew Brzezinski yesterday with trying "to whip up animosity" with "the false story" that the Vietnamese-Cambodian conflict is a Soviet-Chinese "proxy war."

Tass, the official Soviet news agency, snapped back at President Carter's adviser that "the assistant to the U.S. President obviously passes off the desirable for the real."

Brzezinski said in the closing moments of a television interview Sunday on "Face the Nation" (CBS, WTOP) that he regards the fighting between the Vietnamese and the Cambodians "as the first case of a proxy was between China and the Soviet Union."

Other American officials do not go that far, and the State Department yesterday avoided such a characterization, because a proxy war literally means a conflict fought on behalf of other nations. Brzezinski evidently did not quite mean that either.

In the broadcast Sunday Brezezinski went on to say that "the Vietnamese are, clearly, supported by the Soviets, politically and militarily, and the Combodians are supported politically and, perhaps militarily, by the Chinese."

This, however, does not necessily signfy a proxy war, and Brezezinski himself said "I think the Vietnamese Cambodia conflicts have a reality of thown." He said the United States does not have intelligence information that either Soviet or Chinese miltary advisers are participating in the border conflict.

But, Brzezinski said, the fact that Camodia is "claiming" that Russian advisers are present "is itself politically important, even if untrue." He said "the larger, international dimension of this conflict speaks for itself."

Tass said through a "political comntator" yesterday, that the Cabodian-Vietnamese conflict "causes the sincere anxiety of all those who for a lengty period of time had followed the heroic struggle of the peoples of Indoc for liberation, against imperialist aggression."

Claims by "bourgeois propaganda" that foreign advisers" are present in the fighting are "groundless," Tass said - omitting the fact that it was Cambodia that raised that charge and claimed that Russion-speaking voices were heard on the battlefield.

Tass then fired a thinly veiled Soviet shot at China, saying, "Reports coming in from the rea of the conflict indicate that it is precisely Peking that is giving Kampuchea (Cambodia) both political and military support.

Charged Tass: "By putting into circulation the false story about 'a proxy war between China and the Soviet Union' certain circles in the United States demonstrate their desire to see that Soviet-Chinese relations remain spoiled, still better - tense, and count to posion the international atmosphere."

A spokesman for Brzezinski, Jerrold Schecter, said yesterday, "We're not going to debate the Soviets on this matter. The Cambodian statements peak for themselves."

Other American officials yesterday steered clear of the "proxy war" terminology, while recognizing the abundant evidence that the Soviet Union and China are favoring opporing sides in the conflict. The fighting, officially unadmitted until New Year's Eve, appears to be a major embarrassment to Moscow, Peking and Hanoi, if not to the xenophobic leaders of Cambodia.

State Department spokesman Hodding Carter III said, "We basically are not in a position to have a clear and easy answer about what is going on." Asked if he was backing away from Brezezinski's "proxy war" language, Carter said, "I'm really not dealing with the interpretation."

Later, after discussion with Schecter, Carter said, "The main point is that the conflict involves age-old animosities and a border conflict. Overlying it are great-power rivalries."

Cambodia claims it has inflicted nearly 30,000 casualties on intruding Vietnamese forces since last September. Vietnam, admitting no incursion into Cambodia, claims it is repelling Cambodia troops that crossed its border and inflicted "thousands" of civilian casualties.

Cambodian authorities in Peking yesterday contended that their troops reached the northern border of the "Parrot's Beak" salient jutting into Vietnam, and "fully liberated" that region. If true, that could mean Vietnamese troops there have a withdrawn, an American specialist suggested, because Vietnam has vastly superior forces. But all accounts from the war front are totally suspect by outside specialists.