The Soviet Union is expected to deploy an additional 60 to 110 SS20 medium-range nuclear missiles, based on the construction of new launching sites, Assistant Secretary of State Richard Burt said today.

Burt, briefing reporters after a meeting of NATO's Special Consultative Group, which reviews U.S.-Soviet negotiations on medium-range nuclear weapons, declined to predict over what period the new SS20s would be deployed.

"We are entering a negotiating period. I think it is dangerous to speculate," Burt said, referring to the talks in Geneva in January between Secretary of State George P. Shultz and Soviet Foreign Minister Andrei Gromyko on resuming nuclear arms control negotiations.

"It is possible at this stage to suggest that the ultimate number of SS20 deployments, based on what we know about new base construction, is likely to range between 450 and 500," he said.

The growing number of SS20s, each carrying three nuclear warheads, was one reason NATO decided in 1979 to deploy new cruise and Pershing II medium-range missiles in Western Europe. The number of SS20s reportedly deployed had remained stable this year at 378. But Defense Secretary Caspar W. Weinberger announced earlier this month that the number had increased to 387. He also said the Soviets had increased Soviet production efforts to enlarge their SS20 force.

The number of SS20s is watched closely by NATO officials, in part because one member, the Netherlands, said last June that it would deploy its share of 48 cruise missiles only if the number of SS20s had increased from 378 as of Nov. 1, 1985, when it will make a final decision on the question.

Burt also indicated that the U.S. negotiating strategy for Geneva was still under discussion. "To the extent that we could today, we tried to explain our perspective on these meetings in Geneva and listen to the views of our allies," he said. He promised the "closest possible consultation" with Western European governments if the Geneva talks produce further negotiations.