A senior Soviet spokesman said today the Reagan administration's threat to disregard the SALT II treaty will have a "negative" impact on arms talks in Geneva.

Deputy Foreign Minister Alexander Bessmertnikh said President Reagan's recent statements cast doubt on the sincerity of the Americans' pursuit of new arms control agreements.

At a press conference here today, Bessmertnikh described SALT II as a "solid platform" on which to build future agreements. SALT II, signed in 1979, was never ratified by the U.S. Senate although its limits have been tacitly observed by both superpowers.

"Now this platform is being blown up," he said, and the overall impact on U.S.-Soviet relations "does not lead to optimism."

He repeated the Soviet assertion that recent U.S. actions have undermined the conditions necessary for a second summit meeting between Reagan and Soviet leader Mikhail Gorbachev.

But he said Moscow's dismay with U.S. actions will not keep the Soviet Union from dealing with the present administration. "We are dealing with an administration elected by the American people and we will deal with them as long as they are in the White House," he said.

At today's press conference, Bessmertnikh and Soviet Chief of Staff Sergei Akhromeyev said Reagan's renunciation of the SALT II treaty last month proved that Washington has a basic aversion to arms control.

Bessmertnikh said it also disproves the Reagan administration's insistence that the development of a space-based missile defense system will preclude the need for offensive nuclear weapons.

A formal U.S. withdrawal from SALT II would necessarily entail a Soviet response, the Soviet officials said. Bessmertnikh said the Soviets "will take necessary practical steps to preclude the breaking down of nuclear and strategic parity."