The United States cautiously welcomed a Soviet decision to begin destruction of its chemical weapons stockpile this year but said Moscow still lags behind Washington in the battle to rid the world of the lethal arms.

Soviet Foreign Minister Eduard Shevardnadze made the surprise announcement Sunday in a speech to a 145-nation conference on chemical warfare that resumed Monday.French President Francois Mitterrand organized the chemical weapons meeting at UNESCO headquarters in Paris to strengthen a 1925 Geneva protocol prohibiting chemical warfare and to speed up a ban on production of chemical weapons already under discussion in Geneva.

"We shall soon complete the construction of a chemical weapons elimination facility at which we shall proceed immediately to the elimination of our chemical weapons stockpiles," Shevardnadze told the conference.

"Speaking on behalf of my country's leadership, I would like to state from this high rostrum that the Soviet Union . . . will begin in 1989 the elimination of its chemical weapon stockpiles at a facility built for that purpose," the Soviet envoy said.

Secretary of State George Shultz, who met Shevardnadze for some 90 minutes Sunday to discuss the lethal weapons, said: "We welcome that (Soviet) statement."

But he added, "I think it's worth noting that while they are building a plant (to destroy chemical weapons), we have a plant in being.

"So this is something that prospectively they will do when their plant is completed," Shultz told a news conference. "We are proceeding to destroy the unitary stocks as we build much safer binary stocks."

He said the United States had "invited people to come and see" the U.S. plant where "unitary" chemical weapons - toxic by themselves - already are being destroyed. Binary weapons become toxic when two safe chemicals are combined.

Since 1982, the United States has been destroying its unstable, older chemicals at a plant in Tooele, Utah, said a senior U.S. official who declined to be identified, at a briefing called by the U.S. Arms Control and Disarmament Agency.

The official said that the Soviets were getting attention because of their new policy of openness.

The Soviet Union officially admits to 50,000 tons of chemical weapons in its arsenal of 19 varieties of chemical weapons and says it is approximately the same as the U.S. stockpile. It has dismissed as "fantastic" Western estimates of500,000 tons of chemical weapons.

Shultz also said the Soviets had promised to investigate U.S. allegations Libya is building a chemical weapons plant in Rabta, 35 miles southwest of Tripoli. Libya maintains the plant is to make pharmaceuticals.

Shultz said the U.S. delegation made a 10-minute presentation to Shevardnadze and his delegation of material "not designed to be some sort of conclusive and detailed examination, obviously, but to make it clear that this is a very serious proposition.

"He (Shevardnadze) has said - and I believe - that he and his colleagues will look into the matter and that is what we wanted," Shultz said.

Shevardnadze said Monday documents Secretary of State George Shultz showed him during a chemical warfare conference did not prove Libya was building a chemical weapons plant.

The Soviet official told reporters at Orly Airport that Shultz had showed him plans of the Rabta factory that U.S. officials say is being built to produce chemical weapons.

"I told him that this was a serious accusation and that it has to be proven," Shevardnadze said.