The Soviet Union publicly rejected the U.S. position on Moscow's troops in Cuba yesterday as negotiations began here in an effort to resolve the politically sensitive dispute.

Departing abruptly from its circumspect treatment of the matter, the Soviet Communist Party newspaper, Pravda, strongly attacked Washington's recent disclosures about a Soviet combat brigade as "totally groundless."

Both the prominence of the Soviet statement, a page one editorial in today's Pravda, and its distribution by the official news agency, Tass, within minutes after the first U.S.-Soviet negotiating session in Washington, added to the impact of the tough declaration.

With a number of U.S. senators saying that the embattled stratetgic arms limitation treaty (SALT II) cannot be ratified without withdrawal of Soviet combat troops from Cuba, the stakes in the dispute are extremely high for both the Carter administration and the Soviet leadership of President Leonid Brezhnev.

The Pravda editorial did not specifically say that Moscow will refuse to withdraw its troops or accommodate Washington's concern in some other fashion, but it left the impression that this is the case.

The statement described Soviet forces in Cuba as a training unit that has not changed in function or number since it was introduced in 1962, the year of the Cuban missile crisis.

Saying that the Soviet personnel were there at Cuba's request solely to aid that country's defense, Pravda declared that "any attempts to restrict this right [of defense] are in crying contradiction with accepted norms of international intercourse and are absolutely unfounded."

Moscow's unexpected blast after 10 days of gingerly treatment of Washington's charges, cast a pall over the future of the negotiations that began yesterday with a two-hour session at the State Department between Secretary of State Cyrus R. Vance and Soviet Ambassador Anatoliy F. Dobrynin.

The meeting ended with no public statement in Washington from either side, and unusual efforts by both Vance and Dobrynin to keep the tenor as well as the substance of their talk under wraps. The State Department announced that another meeting will be held today or Wednesday, but officials observed an unusually tight ban on other comment, at Vance's order.

The veteran Soviet ambassador, who came to Washington the year of the Cuban missile crisis and who recently became dean of the foreign diplomatic corps here by reason of his long service, had only a wave for waiting journalists as his limousine headed for a scheduled basement entrance. He evaded the press altogether on the way out by taking a side door.

The publication by Tass of the Pravda editorial nearly simultaneously with the Vance-Dobrynin meeting suggested that in tone, if not in detail, it represented the Soviet position in the negotiations.

On the other hand, the refusal of Dobrynin to see reporters and the omission of any mention of the U.S. Soviet talks in the editorial left some room for hope that the Russians will be more flexible in their diplomatic maneuvering than in their public position.

The State Department, taking a cautious position, said only that the Pravda editorial is "not a helpful contribution to the resolution of the problem." A department spokesman refused any further comment.

The U.S. position is complicated by the government's statement that the Soviet force in Cuba has been there for several years and possibly as long as 17 years, and that it poses no military threat to the United States as presently equipped.

On the Soviet side, the involvement of a close ally which has been threatened and invaded in the past by the United States poses an extremely touchy situation. Cuba's Fidel Castro was reported to have been furious when Soviet missiles and aircraft were withdrawn in 1962 without his agreement, and relations between Moscow and Havana cooled for several years thereafter.

Some Moscow press reports had scoffed at the U.S. charges of a Soviet combat brigade in Cuba, made public by Senate Foreign Relations Committee Chairman Frank Church (D-idaho) on Aug. 30 and announced officially by the State Department the following day.

Moscow had avoided authoritative or prominent comment until yesterday, however. And earlier press commentaries said little about the role of the Carter administration in disclosing information and expressing concern about the Soviet brigade, centering attention instead on Church and other members of Congress.

The Pravda editorial, headlined "Who and why needed this?" heaped criticism on the U.S. government for "adding fuel to the fire" by "alarm-spreading statements" rather than explaining what Pravda called "the true state of affairs" about Soviet troops in Cuba.

The Soviet party organ also sharply criticized the role of the U.S. press, calling it "American propaganda media."

Pravda did not accuse President Carter personally of fanning up anti-Soviet propaganda, but asserted that "it is not by chance that all this outcry is being used by those circles in the U.S. that are trying to prevent ratification of the SALT II treaty and in any case to complicate its ratification." It also suggested that the charges were timed to embarrass Cuba while Havana was host to the recent conference of non-aligned nations.

Unlike some earlier press reports and informal remarks of some Russians, the Pravda editorial made no mention of the U.S. naval base at Guantanamo Bay, Cuba, which is maintained under a 1903 lease despite the strong opposition of the Cuban government.

Suggestions of an American withdrawal from Guantanamo in exchange for Soviet withdrawals from Cuba were rejected yesterday by State Department spokesman Hodding Carter. He said Guantanamo is "not negotiable" and that, in general, any attempt to equate one withdrawal with another "will not be fruitful."