The Soviet news agency Tass said today that the United States, by siding openly with Britain in the South Atlantic dispute, "bolstered the aggressiveness" of Prime Minister Margaret Thatcher's government and led it to "escalate military pressure on Argentina."

Saying the conflict between Britain and Argentina was a part of the Reagan administration's "global imperialist line" for extending "U.S. and NATO influence and presence wherever possible," the Soviets argued that the United States has betrayed Latin American nations by refusing to honor its commitments under the 1947 Rio Treaty.

In two Tass commentaries, Moscow appeared to try to exploit the Falklands crisis to depict the United States as the instigator of the conflict while shedding its own image in Latin America as a superpower bent on fomenting local revolutions.

The unfolding Falklands crisis and Moscow's insistence that it is a matter of decolonization seems to have placed the Soviets on the side of most Latin Americans. Analysts here said the Soviets appear confident that they stand to make long-term political and propaganda gains in the region regardless of the conflict's outcome.

According to diplomatic analysts here, the crisis was a gift to Moscow. It diverted the world's attention away from the simmering Polish crisis and tied up the largest European NATO navy in a war in the South Atlantic against a traditional U.S. ally.

According to this view, very little could happen in the British-Argentine conflict that would go against Soviet interests.

From Moscow's point of view, the Falklands eventually would come under Argentine sovereignty even if the British manage to regain control over the islands.

Whatever the outcome, the Soviets see a deterioration of U.S. positions in Latin America in the long term. Moreover, a weakened Argentina is expected to become an even more eager seller of grain and meat to the Soviet Union.

There has been speculation among Latin diplomats here that the Argentine junta may seek Soviet assistance in case of a larger conflict and possible defeat by the British.

Such a request is regarded as unlikely, however, and the Soviets at this stage do not seem prepared to go beyond rhetorical and diplomatic support for Buenos Aires.

In one of its commentaries, Tass said the Soviet Union is strongly opposed to colonialism and "is convinced that the restoration of the colonial status of the islands sought by London and those supporting its military venture is inadmissible and that the dispute should be settled in accordance with the well-known U.N. resolutions by peaceful means, at the negotiating table."

Although Soviet commentaries and dispatches denounced "the imperial ambitions" of the Thatcher government, there were no indications that the Soviet Union was considering the possibility of direct involvement in the conflict.

What the commentaries did, however, was give Moscow an opportunity to score propaganda points in an area traditionally considered Washington's backyard.

Tass said Washington often tried to demonstrate that "the United States was the most loyal friend and dependable ally of Latin American countries and that it was prepared, if need be, to honor the commitments assumed under the so-called Rio Treaty and take part in the collective defense of those countries," Tass said.

It noted that Reagan mentioned the 1947 treaty during his recent speech before the Organization of American States when he outlined plans to aid Central American and Caribbean countries.

"But now that there has emerged a situation in which one of the Latin American States--Argentina--is indeed threatened with the British mailed fist, the Washington 'friend' not only has forgotten the Inter-American Assistance Treaty, not only pointedly refused to subscribe to the decision of the consultative conference of the OAS foreign ministers, which called on Britain to put an end to military operations, but openly stated its support for London."

Observers here say the price in lives and materiel Britain will have to pay to achieve success in the Falklands could rebound against the Thatcher government, which the Soviets would like to see collapse.

Should the conflict escalate into a major war, the Soviets expect United Nations intervention. Moscow expects that most Third World countries would side with Argentina.