The Reagan administration has told Moscow it wants to inspect three old Soviet radars to determine whether they have been deployed at new locations in violation of the 1972 Antiballistic Missile (ABM) Treaty, U.S. officials said yesterday.

The request came in response to a Soviet invitation last month for a team of American inspectors to visit one of the radars, which was moved 2,000 miles earlier this year from a missile test range at Saryshagan, in south-central Russia, to an electronics plant in the city of Gomel, north of Kiev.

Last Thursday, the administration decided to accept the Soviet offer and also seek access to an associated radar at Gomel and a third radar near Moscow.

Defense Department officials and conservative senators have charged that the radars, known to specialists as "Pawn Shop" and "Flat Twin," are deployed illegally because they are no longer at a designated missile test range, as the ABM Treaty requires.

The allegations have taken on political significance because conservatives have cited the radar deployments as evidence the Soviets cannot be trusted enough to comply with a U.S.-Soviet agreement on intermediate-range nuclear forces (INF) to be signed by President Reagan and Soviet leader Mikhail Gorbachev at the Dec. 8-10 summit in Washington.

Conservatives have also argued that the deployments bolster the administration's previous assertions that the Soviet Union "may be preparing an ABM defense of its national territory," although this theory is disputed by some officials.

"Our position is that these radars, if deployed outside of areas permitted by the ABM treaty . . . would constitute a violation of the treaty," State Department spokesman Charles E. Redman said. "An examination along the lines we have indicated could alleviate our concerns regarding these Soviet activities."

Other officials said the specific purpose of the on-site inspection would be to verify Soviet claims that electronic equipment associated with the Pawn Shop radar was never installed after the building housing the radar was moved to Gomel. It could also verify Soviet claims that the Flat Twin radar is not fully reassembled and will be incapable of supporting an ABM system.

First erected at Saryshagan in the early 1970s, the radars were designed to track incoming ballistic missiles and guide interceptors to destroy the missiles in a nuclear explosion, according to U.S. intelligence experts.

The Pawn Shop radar is mounted on a low trailer, with associated equipment normally located in additional trailers nearby. The Flat Twin radar, in contrast, has a sheer face and is roughly three stories high.

Both are considered an outmoded technology that can be readily overwhelmed in a U.S. nuclear attack. Hundreds of more modern versions would be needed in a credible ABM system, officials said.

Defense Department officials also said the radars in Gomel are situated so close to other large buildings that using them would severely harm nearby personnel. No Soviet missile interceptors have been deployed nearby to use in conjunction with the radars.

The administration debated for weeks before proposing the on-site inspection, with the Joint Chiefs of Staff expressing reservations that it would lead to a similar Soviet request for access to a sensitive military facility in the United States.

Meanwhile, in Geneva yesterday chief U.S. negotiator Max M. Kampelman and his Soviet counterpart, Yuli Vorontsov, decided to extend their talks through tonight in an effort to work out final details of the pending INF treaty.

In brief appearances during the course of the discussions, both officials expressed confidence that remaining disputes over measures to verify treaty compliance would be worked out and the treaty would be finished on time.