The Soviet government belatedly acknowledged to its people today that "basic agreement" has been reached with the United States on a new strategic arms limitation treaty.

The official announcement by the government news agency Tass came 18 hours after Secretary of State Cyrus Vance's announcement of the agreement at a White House press briefing yesterday. While the Soviet media remained officially silent until late this afternoon, the news of the accord was broadcast into the country by Western radio stations.

Although treatment of major news events by Soviet media is notably capricious, the official silence about the strategic arms agreement puzzled and annoyed some Western diplomats. Several brushed aside suggestions that the Kremlin was merely exercising its usual caution. "If they agreed to it, why withhold announcing it?" one West European asked in exasperation.

Quoting Vance, Tass said, "He announced that a basic agreement was reached between the Soviet Union and the U.S. on the main question of the substance of the negotiations on limitations of strategic armaments. . . . The secretary said the U.S.S.R. and the U.S. were now on the threshhold of signing a new treaty on SALT, which will mark practical restrictions in the nuclear arms race."

Some diplomats said the Soviet delay and gingerly description of the agreement stems from Kremlin reluctance to make a full disclosure until the exact details of the summit meeting between President Carter and Soviet President Leonid Brezhnev can be announced. "They want it completely nailed down first, then they can feel free to lent Brezhnev's name to it," said one.

At the same time, some suggest Tass has taken pains to qualify its report out of Kremlin concern that the U.S. Senate may eventually reject the pact, dealing a major blow to the prestige of Brezhnev and his politburo, who have made SALT a foundation of bilateral detente.