GENEVA, JUNE 9 -- The Soviet Union indicated for the first time today that it is willing to accept mandatory inspection of suspicious activities within its territory as part of a comprehensive ban on nuclear testing.

The Soviet Union has never before agreed with the western position at a 40-nation disarmament conference in Geneva that compliance with nuclear arms treaties must be verified by mandatory and quick "challenge" inspections to ensure against cheating.

The Soviet move comes as U.S. intelligence officials are demanding an exemption for their facilities from a requirement for mandatory inspection of suspicious activities under a medium-range missile treaty with the Soviet Union, U.S. officials said. A decision on the intelligence community demand is pending at the White House.

{In Washington, a State Department official described the Soviet announcement as "a significant development that could make a big splash."}

Vladimir Petrovsky, a deputy Soviet foreign minister, said in presenting a detailed nuclear testing proposal to an opening session of the disarmament conference here that he wanted "to stress that the state which has received a request for an on-site inspection will be obliged to allow unconditional access to the location designated in the request.

"In other words, the issue at hand is mandatory, not voluntary, inspections."

Petrovsky later told reporters that inspections would have to be carried out quickly, "surely, surely in less than one week."

Petrovsky also stressed that the Soviet Union was ready to resume its nuclear test moratorium "even today" if the United States would go along. The Reagan administration has refused to join a moratorium or enter into negotiations with the Soviet Union on a full test ban, saying tests are essential to maintain atomic arsenals. Moscow declared a unilateral test moratorium in August 1985 that lasted 18 months.

Under Petrovsky's plan, which he said was submitted on behalf of Warsaw Pact members, the two sides would agree immediately to restrict each of their atomic tests to one kiloton, the equivalent of 1,000 tons of TNT, and to limit the number of tests a year.

Two unratified treaties of the 1970s set a limit of 150 kilotons. No treaty has ever set a limit on the number of tests that each side can carry out. Petrovsky also indicated that, despite the new position on mandatory inspections of suspected nuclear tests, the Soviets would continue to resist U.S. demands for mandatory inspections under an international treaty banning production and stockpiling of chemical weapons.

He said chemical arms negotiations were in the "home stretch" and repeated Moscow's belief that a treaty could be signed this year. But he indicated that the U.S. demand remained a major stumbling block because, unlike nuclear test sites which are confined to just a few locations, chemical stocks and production facilities can be moved easily and quickly from one site to another. As a result, the U.S. demand in the chemical negotiations would subject many more Soviet facilities to quick inspection than the agreement on nuclear testing.