Soviet leaders continue to attack President Reagan's "Star Wars" missile defense system and call for the "demilitarization of space," but they suddenly have gone silent on their two-year campaign for a moratorium on testing and deployment of antisatellite weapons, according to sources inside and outside the administration.

A top administration arms control official, who has worked to answer the Kremlin campaign, said yesterday the silence is "significant," but "we don't know what it means."

Both Star Wars and the antisatellite program (ASAT) involve weapons in space. Star Wars, using either land-based or space-based installations, would attempt to shoot down missiles in flight. ASAT would aim to destroy satellites that help target those missiles and provide spy information and communication.

Satellites would play a key role in the Strategic Defense Initiative -- Star Wars -- by providing information to help destroy ballistic missiles before their warheads could hit targets on land.

The Carter administration unsuccessfully sought to negotiate an antisatellite weapons treaty with the Soviet Union in the late 1970s. At that point, the Soviets had a rudimentary system and the United States was beginning to develop a more sophisticated weapon. The Reagan administration refused to resume the talks, saying an agreement could not be verified.

Soviet leaders, beginning in 1983, began a campaign for a halt in U.S. testing of ASAT weapons. As late as December, Soviet President Konstantin U. Chernenko called for such a ban.

Nonetheless, when Soviet Foreign Minister Andrei A. Gromyko met in Geneva with Secretary of State George P. Shultz on Jan. 7-8, he never mentioned the subject.

"We thought it was unbelievable," one official said.

When Gromyko held his two-hour news conference Jan. 13 to discuss the resumption of arms talks, he never mentioned antisatellite weapons. "That convinced us they had made a decision to drop the subject," one official said.

Some Reagan officials say they now believe that the Soviets have decided to focus all their attention on Star Wars research. Others say they believe that Moscow has plans to resume testing and development of its antisatellite systems.

On Capitol Hill, one defense specialist pointed to a new delay until June in the first major test of a new U.S. antisatellite system and suggested "a private deal may have been struck. They stop talking about a moratorium and we hold off testing."

Top Pentagon and Arms Control and Disamament Agency officials believe, as one said, that the Soviet propagandists "only want to go after the big one," the Star Wars program.

Another top official, however, said he believes the silence indicates that the Soviets "have decided to resume testing themselves so they will have weapons to destroy a future U.S. space-based ballistic missile defense system."

Retired Lt. Gen. Glen Kent, senior analyst with the Rand Corp., and a specialist on strategic systems, said yesterday, "The Soviets suddenly got the message . . . if the United States deploys defenses in space, they have got to keep antisatellite weapons as an ace in the hole to disable those defenses."

Defense Secretary Caspar W. Weinberger, in his annual posture statement released earlier this week, said that last year's proposed U.S.-Soviet arms talks in Vienna failed because Moscow "demanded that we institute a moratorium on antisatellite tests before opening any talks." That, Weinberger said, was "unacceptable" because it would give the Soviets "a weapons monopoly."

For 10 years, the Soviets have had a rudimentary antisatellite weapon that is fired at a satellite in orbit. After several orbits of its own, guided by an onboard radar, the Soviet weapon draws near its target and explodes.

Since 1977, the United States has been working on a system that would be carried aloft on a rocket fired from an F15 fighter. Unlike the Soviet system, the U.S. weapon would be aimed directly at the Soviet satellite and guided directly into it by a homing device.

Electronic and mechanical problems have delayed U.S. test schedules by almost two years. The first test, which could have occurred after March 1, was postponed until June, according to sources.