GENEVA, AUG. 6 -- Soviet Foreign Minister Eduard Shevardnadze today accused the United States and West Germany of blocking a proposed arms control accord by using a "legal sham" to justify excluding 72 Pershing IA nuclear warheads from the deal.

Senior U.S. and West German officials reaffirmed their position that the U.S.-controlled warheads are outside the scope of the proposed treaty because they are poised on West German missiles and thus remain exempt from a bilateral U.S.-Soviet accord.

U.S. national security adviser Frank C. Carlucci assured top West German officials at meetings yesterday and today that Washington is sticking to its stance, U.S. Ambassador to West Germany Richard Burt said.

Shevardnadze, in a speech to the 40-nation Conference on Disarmament, also announced what appeared to be a significant Soviet concession on how to verify a planned international convention to ban chemical weapons.

He said the Soviets had accepted the principle, backed by the United States, that any country accused of cheating would be subject to "mandatory challenge inspections without right of refusal."

Max Friedersdorf, U.S. ambassador to the disarmament conference, welcomed Shevardnadze's statement on chemical weapons as a "constructive step." But another U.S. official expressed caution, noting that Shevardnadze had not said that any suspicious site should be subject to inspections at any time, as the United States has proposed.

Shevardnadze's toughly worded comments on the Pershing IAs appeared designed to "intimidate" the West German government, senior U.S. officials said. Bonn would like to keep the weapons but does not want to appear to be obstructing an arms control accord.

In a potentially significant omission, however, Shevardnadze did not rule out the possibility that the Pershing IA issue could be resolved by a pledge by the United States or West Germany not to replace the antiquated missiles with more modern weapons. He did not comment on the no-modernization suggestion, which has been widely mentioned as a potential compromise.

The Pershing IA issue is one of the last remaining obstacles to a U.S.-Soviet treaty to dismantle all of the superpowers' intermediate-range missiles and warheads.

The Pershing IA, with a range of 460 miles, is in the category of weapons to be scrapped. But the Americans contend they would betray their West German allies and set a precedent for undermining other U.S.-allied nuclear cooperation programs, if they yield to the Soviets by destroying the Pershing IAs' warheads.

Shevardnadze challenged the U.S.-West German contention that the weapons cannot be discussed at the U.S.-Soviet negotiations here because they are under joint control.

"There can be no nuclear weapons on earth that are no one's weapons. It is extremely dangerous to create such a legal sham," Shevardnadze said.

He said that West Germany would be in violation of the Nuclear Nonproliferation Treaty if the weapons, including their warheads, are West German.

Chief U.S. arms negotiator Max Kampelman welcomed a statement by Shevardnadze that Washington and Moscow are close to an intermediate-range missile treaty. But Kampelman said that he was "troubled and angered" by Shevardnadze's "intimidating" attitude toward West Germany.

Kampelman, echoing other recent U.S. comments on the Pershing IAs, left open the possibility that the problem could be solved outside the Geneva talks. He hinted that he wants the Soviets to take the lead on the issue, whereas the Soviets recently have suggested that they want Washington or Bonn to propose a compromise.

"I'm sure that the Soviets are sufficiently creative to find a way to solve this problem without including it in an INF {Intermediate Nuclear Forces} treaty," Kampelman said.

Meanwhile, Ambassador Burt said the West Germans "seemed very satisfied" with assurances from Carlucci about the Pershing IAs.

Carlucci met yesterday with West German Chancellor Helmut Kohl at Kohl's vacation retreat in Austria, and with Foreign Minister Hans-Dietrich Genscher at the Bavarian resort of Berchtesgaden. Today Carlucci met with parliamentary leaders in Bonn.

"Carlucci reiterated our position that the Pershing IAs are not on the table at Geneva," Burt said in a telephone interview.

Burt also said that the Pershing IAs, as a form of U.S.-allied cooperation, "have no place in a bilateral U.S.-Soviet dialogue," even outside Geneva. His comment appeared to reinforce recent private suggestions from U.S. and West German sources that a compromise, if it emerged, would have to be the product of unilateral policy declarations rather than the result of bargaining.

Shevardnadze's statements on a chemical weapons accord moved the Soviets closer to the United States on the key issue of verification.

Shevardnadze invited the chemical weapons negotiators to visit a major Soviet chemical arms facility at Shikhany on the Volga River "to see standard items of our chemical weapons and observe the technology of destroying chemical weapons at a mobile facility."

The Soviets later will invite experts to a specialized chemical weapons destruction plant being built near Chapayevsk in Soviet Asia, Shevardnadze said.