A Kremlin official warned again today that deployment of U.S. nuclear missiles in Western Europe on Soviet targets could upset the military balance in the East-West theater and draw a tough Soviet countermove.

Vadim Zagladin, first deputy chief of the Communist Party Central Committee's international department, asserted that there is "a rough balance of forces in Europe . . . But if [longer range] American rockets were introduced, this would change the present situation, we are agreed."

Zagladin implied that the Soviets would move quickly to expand their own nuclear striking forces in response. "It is necessary to do everything to preserve the balance of forces on the present level, or even lower," he said.

Soviet President Leonid Brezhnev said Monday that new Western deployment of nuclear rockets in Europe would be "a dangerous game [played] with fire."

The Soviets are growing increasingly apprehensive that a long-proposed deployment of advanced Pershing II medium-range missiles in West Germany and in one other Western allied country may be approved at the NATO Council session in December. Brezhnev is scheduled to deliver a major foreign policy speech Saturday in East Berlin, parts of which may deal with the allied deployment plan.

Zagladin made his comments at a press conference here marking the end of a five-day session of a Socialist International Working Group on Disarmament. Several members of the group, composed of representatives from 42 noncommunist countries, said they believed Brezhnev will have more to say about the missiles. They refused to divulge the details of conversations they held with the Soviet president and party leader Monday in the Kremlin, but they did not reject suggestions that theater nuclear strength will be a Saturday topic for the 72-year-0ld Soviet leader.

Brezhnev arrived today in East Germany for a visit marking its 30th anniversary.

It seems likely as well that Brezhnev will talk about President Carter's decisions this week to beef up U.S. forces in the Caribbean in response to Soviet troops in Cuba, which the administration alleges form a combat brigade. The Soviets have made no authoritative comment on Carter's move, which came after a month of fruitless negotiations between the governments on the issue, which has slowed Senate ratification of the second strategic weapons limitation treaty.

Another Soviet official at today's press conference separately called on the West to at least accept Soviet figures on the size of Warsaw Pact ground troop concentrations so that the two sides can progress toward mutual troop reductions.

Valentin Falin, first deputy chief of the Central Committee's international information department, said the West had "invented" the issue of overall combat troop numbers as a means of stalling the Vienna talks on mutual force reductions.

He said there are 305,000 Warsaw Pact ground troops in Central Europe; the West estimated there are 150,000 more. Falin said East and West have sophisticated national technical means to verify the precise numbers of troops on each side. The Vienna talks have been bogged down in a number of disputes including one on whether Soviet assertions of their troop numbers are accurate.

"With the means of detection currently available, there is no point in giving incorrect figures. The lie would quickly be found out, "Falin said. We believe it is time to stop such useless discussions in Vienna and move on to talking about major problems."

The Socialist International issued a joint statement with the Soviets calling for quick implementation of the SALT II treaty, opening of SALT III talks and an early conclusion of the Vienna troop talks.