American and Soviet negotiators are expected to announce today that they have surmounted some of the obstacles impeding a nuclear arms control agreement, improving the overall climate for a new SALT pact.

Soviet President Leonid Brezhnev this morning is scheduled to join the closing round of three days of negotiating with Secretary of State Cyrus Vance.A communique reporting that encouraging progress has been made in the talks, pointing hopefully toward another series of negotiations in the United States late next month, is expected to follow.

By deliberately setting modest goals for the nuclear negotiations here, with emphasis that the tone of the meetings is even more significant than the details, Vance sought to avoid a backfire from unfulfilled expectations.He has apparently suceeded.

The Soviet Union, barring any sudden new move by Brezhnev - and none is anticipated - appears equally satisfied to settle for limited advances. It has been assured that the United States is determined to complete the negotiations as quickly as possible.

In the earlier portion of this overseas mission, in Africa last week, Vance similarly proclaimed limited goals and therefore maintained that he had achieved some progress. This is now the emerging pattern in administration foreign policy: modest claims and marginal advances.

The negotiating teams headed by Vance and Soviet Foreign Minister Andrei Gromyko met for only two hours last night. They discussed the strategic arms limitation talks (SALT) ging and unproductive East-West negotiations on reduction of conventional armed forces in Europe and plans for the special session of the U.N. General Assembly on disarmament starting at the end of May.

There, and perhaps during a Gromyko visit to Washington also, it is hoped that the Soviet and American positions on a new nuclear pact to run to 1985 can be nearly completed, to be followed by a subsequent Brezhnev-Carter summit meeting to wrap up the negotiations.

The Brezhnev-Vance talks today, in restricted session, are intended to lay down that projected pattern. The Kremlin has accused the Carter administration of vacillation and hesitation in the nuclear arms control talks in the recent past.

U.S. spokesman Hodding Carter told reporters last night that "I do not expect that all of the SALT issues will be resolved" before Vance leaves Moscow. But the spokesman again described the attitude in the Kremlin talks as "good and useful."

American officials said the incident involving the downing of a South Korean airliner in the Murmansk sector of the Soviet Union has not entered the Kremlin talks with Vance. Instead, these sources said, that sudden controversy, which aroused far more excitement in Washington than it did in the Soviet Union has been handled by Deputy Secretary of State Warren Christopher.

One of the most troublesome issues on the American-Soviet horizon, the potential future use in Africa of Soviet military power and Cuban troops which were employed in Ethiopia to repel Somali forces intruding into Ethiopia, is being held for discussion in today's Brezhnev-Vance meeting. Brezhnev and Gromyko were both occupied part of yesterday with celebrations of the birthday of Lenin, founding father of the Soviet Union.

A spokesman said Vance in last night's meeting did complain about the Soviet blockage of American news and television transmission of the arrest by force of Irina McClellan outside the American Embassy Thursday. She has unsuccessfully sought an exit visa to join her husband, Woodford McClellan, a University of Virginia professor who married the Russian woman here in 1974.

With all its supposed skill in propaganda warfare, the Soviet bureaucracy on several occasions has made the same blunder of attracting additional attention to unfavorable news by trying to block its transmission.

American officials denied reports that this Soviet action was souring the nuclear arms control talks. Outside issues are not being permitted to derail or disrupt the nuclear negotiations, U.S. officials reiterated.