The Soviet Union will end its unilateral moratorium on testing and deployment of antisatellite weapons if the United States proceeds with a planned test of an antisatellite weapon, the official Soviet news agency Tass said tonight.

The Tass statement is viewed here as a delayed response to Washington's mid-August announcement that it plans to test an antisatellite weapon, and as a climax to the two-week-long Soviet press campaign condemning the U.S. decision.

The U.S. test of an antisatellite weapon launched from an F15 jet fighter against a target in space is expected in about 10 days.

Tass also said the United States has "flatly refused" any measures aimed at banning and limiting antisatellite weapons in the U.S.-Soviet arms talks. The third round is to open Sept. 19 in Geneva.

Tass said that if the United States "holds tests of antisatellite weapons against a target in outer space," the Soviet Union "will consider itself free of its unilateral commitment not to place antisatellite weapons in space."

The State Department, in response, said: "The Soviet threat to deploy an ASAT (antisatellite) system has little practical meaning since they (the Soviets) have had for several years an operational ASAT system. In fact, their operational system goes into space and performs its mission. The U.S. testing program is important in reestablishing the balance in this area."

[The U.S. statement also said the administration was "seeking to discuss a wide range of pertinent issues in the Geneva negotiations."]

The Soviet system uses a missile to hoist a killer satellite into space and then maneuvers it close to an enemy satellite and explodes the killer satellite, with the shrapnel disabling the other craft. The Soviet system is widely viewed by many experts as much cruder than the U.S. system being developed.

The Soviet Union initially reacted to the U.S. decision by stating in a Tass commentary Aug. 21 that its antisatellite test moratorium, imposed in August l983 by the late president Yuri Andropov, was in effect for as long as "other countries, including the U.S., refrain from placing antisatellite weapons of any kind in space."

Western observers here viewed the Soviet decision as the escalation of a Kremlin move to prevent the United States from domination in space weapons. Last month the Soviets proposed an international conference on halting what Moscow calls the militarization of space.

Until recently, Moscow focused its campaign against space weapons on criticisms of the U.S. Strategic Defense Initiative. That program is meant to explore possible space-based techniques for building an antimissile defense system.

But since the United States announced antisatellite test plans, Soviet criticism has broadened to include them. Soviet leader Mikhail Gorbachev blasted the U.S. plan to test an antisatellite weapon in a Time Magazine interview published this week and also in a meeting with U.S. senators yesterday.

The Tass statement called on the United States to "weigh the inevitable negative consequences of its planned tests . . . for political and military stability in the world, and the prospects of the Geneva talks."

The Soviets last tested an antisatellite weapon in June l982, according to a recent U.S. arms control study.

"To our restraint in practical actions," Tass said today, "the American side is responding with a demonstration of its unwillingness to reach agreements, to display mutual constraints."

Soviet officials have not acknowledged conducting any antisatellite tests. Sen. John W. Warner (R-Va.) said after meeting with Gorbachev yesterday that he was "disappointed" that the Soviet leader criticized the SDI plans and the decision to conduct an antisatellite test without admitting that the Soviet Union already had been involved in similar activities.

The wording of the Tass statement leaves it unclear whether Moscow plans to resume testing and deployment of its existing weapon or deploy new ones using laser weapons that Washington believes are under development.

Western observers here interpret the wording of the Soviet declaration of its freedom from a "unilateral commitment not to place antisatellite weapons in space" as a suggestion that it might launch new space-based antisatellite weapons, rather than land-based systems.