West German Foreign Minister Han-Dietrich Genscher today cautioned the Soviet Union and its allies that continued threats protesting the West's plans to modernize its nuclear arsenal in Europe could backfire and worsen prospects for the negotiations on arms control that the Kremlin claims to seek.

Genscher's remarks, issued in a statement here, were the clearest sign yet that the Soviet tactics are irritating the Bonn government. West Germany also is anxious for negotiations to control such weapons but has continued to stress the need for a balance of power between East and West.

"The problem is not the Western modernization program but rather the continuing arms advantage of the East," Genscher said. "We do not want superiority, only a balance," he added.

Genscher called on the Communist bloc to pursue the "constructive" portions of the speech delivered last month by Soviet leader Leonid Brezhnev, in which he combined conciliatory gestures with threats. The West German leader also asked the Communists "not to damage the negotiating climate through threats."

"We want to negotiate," he said, "but no threat will prevent us from deciding upon what is necessary for our own security."

"He also called on Moscow to stop producing the new triple-warhead, mobile SS20 intermediate-range missile as a step toward reaching a negotiated settlement leaving arms at the lowest possible level in Europe. The NATO modernization plan, to be decided upon at a meeting next month in Brussels, is meant largely to offset these new Soviet weapons, which Genscher says the Soviets are adding at a rate of about 50 each year.

Genscher heads the small but crucially important Free Democratic Party that forms the ruling coalition with Chancellor Helmut Schmidths Social Democrats, and he has become the highest ranking German official to take a clear-cut stand in favor of the NATO program, which includes an interlocking arms control proposal. He has therefore become a prime target for the Soviet verbal campaign and was attacked sharply in the Soviet party newspaper Pravda yesterday for allegedly using "pseudo-arguments" to justify NATO deployments.

Two days earlier, Communist East German leader Erich Honecker raised the pressure on Bonn by warning that approval of the NATO plan will not only cause deterioration in East-West relations generally, but also voiced a threat to West Berlin, the allied sector of the divided city.

In recent weeks, East Germany had been especially coridal to Bonn. Last month, an East German Cabinet minister visited West Germany, the first visit by an official of such rank in eight years, and talks were held in an especially good and businesslike atmosphere. Last week, East Germany agreed to drop certain longstanding road tolls on West German cars entering the East, in return for a lump sum payment by Bonn.

Honecker's speech and continuing Soviet threats make it clear, however, that Bonn may have to pay a price for going along with the NATO plan.

West Germany has voiced clear support for the NATO plan. Germany is not only the key because most of the new U.S.-built Pershing II and Cruise missiles would be based here, but also because Bonn has insisted that in not be the only nonnuclear member of the alliance to accept such weapons on its soil. The United States has backed Bonn in this position.

While the West Germans have said they are not linking acceptance of the NATO deployment plan to what any specific country does, the Bonn strategy is to keep the pressure on all the allies and not let anyone off the hook by announcing its intentions too precisely.