The United States and the Soviet Union have agreed to establish "working groups" on such subjects as curbing the spread of nuclear weapons, reducing arms sales, halting all nuclear weapons tests and banning new types of mass-destruction weapons, Secretary of State Cyrus R. Vance said tonight.

Vance described this intermediary step as a sign of "useful and constructive" discussions at the Kremlin today. At the same time, elsewhere in the Kremlin, Soviet leaders weighed their response to the central issue in the Vance mission, President Carter's call for "substantial reductions" in American and Soviet strategic nuclear weapons.

Earlier in the day, Sweden's seismological institute in Uppsala registered a powerful underground explosion in the Semipalatinsk nuclear testing area of western Siberia. The blast had a magnitude equivalent to 5.9 on the open-ended Richter scale, the institute said.

Official silence covered the decisive deliberations by Soviet leader Leonid Brezhnev and his colleagues, which will foreshadow the degree of success or failure in this first venture in detente diplomacy for the Carter administration.

As that internal Soviet process went on, the key subject of Strategic Arms Limitation Talks (SALT) went unmentioned in today's meetings. They were conducted with Soviet Foreign Minister Andrei Gromyko leading the Soviet delegation and Vance heading an American delegation of strategics on weaponry and arms traffic.

Also unmentioned in two meetings today, Vance said, was the prickliest political encounter at Monday's meeting with Brezhnev, his warnings against American "interference" on behalf of human rights in the Soviet Union.Vance said there was "no allusion to human rights today."

The subjects explored today included many topics President Carter listed March 24.

As Vance listed them today, they are: the plodding East-West talks in Vienna on mutual reduction of armed forces in Europe, to get them "out of the doldrums," a total ban on all nuclear weapons testing, now conducted only underground by the United States and the Soviet Union; demilitarization of the Indian Ocean; preventing the spread of nuclear weapons; sales or transfers of conventional weapons and banning new weapons of mass destruction. The last is a special interest of the Soviets.

Vance also said there was some discussion of the Middle East and briefly, of South Africa. He declined to say if he obtained a satisfactory explanation of Gromyko's tempting indication of a shift of Soviet position to circumvent the disagreement over participation by the Palestine Liberation Organization in a reconvened Geneva conference to resolve the Arab-Israeli dispute.

It is up to the Soviet Union to explain its own position, Vance said, adding that he "had the feeling that they wished to play a constructive and active role . . ."

Participating in today's talks on the U.S. side with Vance were U.S. Ambassador to Moscow Malcolm Toon; Paul C. Warnke, director of the U.S. Arms Control and Disarmament Agency; William Hyland of the National Security Council staff, the one veteran in these Kremlin discussions from the era of Henry A. Kissinger; Leslie C. Gelb, director of the State Department's Bureau of Political-Military Affairs, and Lt. Gen. Edward L. Rowny, a SALT expert representing the Joint Chiefs of Staff.

The appearance of Gen. Rowny is especially significant in these meetings. This week is the first time since the SALT negotiations began that a U.S. general has been in the American delegation for such Kremlin talks.