BONN, AUG. 26 -- Chancellor Helmut Kohl pledged today that West Germany would dismantle its 72 intermediate-range Pershing IA missiles, and not replace them with more modern weapons, if the United States and the Soviet Union scrapped all of their own intermediate-range missiles as foreseen under a proposed disarmament treaty.

Kohl's surprise announcement, which was a major concession, appeared to remove one of the last obstacles to a U.S.-Soviet pact to eliminate all ground-based nuclear missiles with ranges of between 300 and 3,500 miles.

Kohl said at a news conference that the antiquated Pershing IAs, which are under joint U.S.-West German control, would be dismantled after all of the U.S. and Soviet weapons were removed under the proposed treaty. He fixed the condition that both superpowers had to adhere to whatever schedule was agreed under the pact for dismantling their missiles.

The treaty, in its current form, calls for the U.S. and Soviet intermediate-range missiles to be removed in stages over five years. As a result, the Pershing IAs would become technically obsolete before the five-year period was up, according to West German officials. The Pershing IAs were deployed in the late 1960s, and were scheduled to be replaced with more modern weapons in the early 1990s.

The initial Soviet reaction was cautiously positive. Radio Moscow called Kohl's statement "a step forward" but added, "A careful analysis is still needed here to assess how far this step goes."

The official Soviet news agency Tass said that Kohl had set "a number of preconditions" for dismantling the Pershing IAs, and it hinted that the Soviets would prefer to see them dismantled earlier than Kohl promised. But Tass did not rebuff the Bonn declaration.

The Soviets had said that they would accept the treaty, now in the closing phase of negotiation at Geneva, only if the U.S.-controlled nuclear warheads on the Pershing IAs were destroyed.

The Pershing IAs, with a range of 460 miles, are in the category of weapons to be dismantled under the treaty. But the United States has said that they are West German weapons and thus outside the scope of the bilateral, U.S.-Soviet treaty.

The U.S. Embassy here had no immediate comment on Kohl's statement, but diplomatic sources said Washington was likely to say it respected Bonn's decision. U.S. officials have said it was up to West Germany to decide how to handle the Pershing IAs.

Kohl took the initiative on the issue largely for the domestic political purpose of casting himself as an active disarmament supporter in advance of state elections Sept. 13 in Schleswig-Holstein and Bremen, government and diplomatic sources said. Kohl's conservative Christian Democratic Union lost ground in national elections in January and in state elections in May principally because its foreign policy was viewed as too right-wing.

"I want to help the American president to bring the Geneva negotiations to a successful conclusion," Kohl said. He also was eager to create a positive climate for an unprecedented visit here by East German chief of state Erich Honecker from Sept. 7-11, the government sources said.

Kohl was under pressure to resolve the Pershing IA dispute from Foreign Minister Hans-Dietrich Genscher, who strongly opposed modernization, and from the opposition, left-of-center Social Democrats. The Social Democrats have called a special session of parliament next week to discuss the Pershing IAs, and the debate there had threatened to expose strains within Kohl's center-right coalition.

In particular, the Social Democratic proposal was aimed at underlining the disagreement between Genscher's moderate Free Democratic Party and conservatives in the coalition. West German conservatives, who have supported keeping the Pershing IAs and replacing them with updated missiles, were notably quiet about Kohl's announcement.

The Christian Social Union, the Bavarian-based sister party of Kohl's Christian Democrats, had no comment, a party spokeswoman in Bonn said. West German television reported that the party was having trouble contacting its leader, veteran conservative Franz Josef Strauss, who was traveling.

By contrast, Genscher said that he was "highly pleased" with the chancellor's statement.

In an apparent bid to appease the conservatives, Kohl called on the Soviet Union and its Eastern European allies to refrain from modernizing their battlefield-range missiles and "to remove" the threat to West Germany posed by such weapons. West Germany has a special interest in seeing cuts in the Warsaw Pact's arsenal of those missiles, which have ranges of less than 300 miles and therefore represent a special danger to West Germany as NATO's front-line state.

But Kohl did not demand concessions on battlefield-range missiles as a condition for scrapping the Pershing IAs. Instead, he set four conditions that amounted to a demand that the Americans and Soviets go through with the intermediate-range missile treaty being negotiated.

The conditions were:That the United States and Soviet Union agree on a global elimination of intermediate-range missiles. That outstanding verification disputes are resolved "in a way that is satisfactory to all concerned." That the treaty is ratified and takes effect. That the superpowers "stick to the agreed-upon schedule for the removal of their weapons systems."

"In that case," Kohl continued, "I am prepared to declare already today that, with the final removal of all Soviet and American intermediate-range missiles, the Pershing IA missiles will not be modernized, but instead dismantled."

Kohl reaffirmed previous statements that it was important for the United States and the Soviet Union to reach an arms control accord this year to leave plenty of time for the Senate to ratify it before the end of President Reagan's term.

Meanwhile, Kohl announced that West Germany was nominating Defense Minister Manfred Woerner to become NATO's secretary general when Britain's Lord Carrington steps down as scheduled in June. Woerner would be the first West German to hold the post.

Norway has nominated former prime minister Kaare Willoch, but NATO sources said Woerner had an early edge because he comes from a more influential country.

Kohl said he had discussed his Pershing IA proposal and the Woerner nomination with senior White House officials "within the last 48 hours." He gave no details.

U.S. sources today confirmed accounts yesterday from West German officials that Secretary of State George P. Shultz and National Security Adviser Frank Carlucci had informed the Bonn government this month that Washington was sticking to its position on the Pershing IAs and was not pressuring the Germans to yield.

The West Germans, however, interpreted that U.S. message to mean that Bonn had to come up with a solution to the dispute or risk being blamed for standing in the way of a U.S.-Soviet arms treaty, according to West German officials.

Some U.S. officials are likely to be unhappy with today's decision, western diplomats said. Many U.S. officials have said in recent weeks that they expected the Soviets to yield on the Pershing IA issue if Washington and Bonn stuck firmly to the western position, and those officials may now feel that Kohl gave up too easily, the diplomats said.

"The message today was that Kohl, essentially for domestic reasons, couldn't afford politically to wait to see whether the Russians would fold," a western diplomat said.

Nevertheless, Kohl's unilateral no-modernization pledge has an advantage for both West Germany and the United States, because it deals with the dispute while technically safeguarding the U.S. position that the Pershing IAs are not subject to negotiation at Geneva, West German and U.S. officials said. As a result, no formal precedent has been set for including other jointly controlled, U.S.-allied weapons systems in bilateral superpower arms negotiations, they said.