The Soviets have begun construction of a large, sophisticated radar facility near Moscow in what U.S. intelligence is calling a "significant" modernization of their antiballistic missile system, according to sources in the administration and on Capitol Hill.

The new radar facility, sources said, appears to be designed to direct a new generation of interceptor rockets that also are beginning to be installed around the Soviet capital.

Administration sources last week were quick to point out that the apparent upgrading of the Moscow ABM system is permitted under the SALT I treaty as long as the new radar remains within specified limits on power and antenna size and interceptors number fewer than 100.

U. S. intelligence has closely followed Soviet tests of advanced radars and ABM interceptors, most of which are carried out at sites in central Russia and western Siberia.

Based on those observations, a Pentagon official said last week that the current radar construction near Moscow was "long overdue" and thus no great surprise.

When completed more than a year from now, he said, the new system "will be far more capable than the one there today."

How capable, however, is difficult to judge. In fact, the United States will not be able to determine whether the new radar stays within the limits of the SALT I treaty power limits until it is turned on.

The Soviet ABM activity is bound to feed that already growing sentiment within the Pentagon and on Capiitol Hill for an expansion of the current U.S. ABM effort.

It also is certain to evoke some political response from the presidential candidates.

The contentious issue is whether the new Soviet radar and interceptors raise a real possibility that the Soviets could "break out" from the SALT I treaty limits and within a brief time put in place a nationwide ABM system.

One pro-Reagan defense specialist who is aware of the Soviet ABM activities said last week that the potential is there "and the U.S. should do something about it."

A Pentagon official responded, "There are things going on to put us in a position to counter Soviet advances, but they are not a big effort."

The Carter administration has increased for fiscal 1981 the roughly $250 million that the United States has spent each year since the signing of the SALT I treaty in 1972.

One defense official said last week that there already are moves within the Pentagon to press for a changeover from general ABM technology research to building a prototype system.

"We are using the Russians as part of the argument that we should build as prototype like they are," he said.

In its recent report on the fiscal 1981 Defense Department budget, the House Appropriations Committee expressed its belief that there should be "some level of system prototype demonstration effort."

The committee echoed the sentiments of many Pentagon officials in saying it was time to develop "integrated systems," but cautioned that they should be "carried out within strict compliance with the ABM treaty."

The committee approved $268 million for the program.

Many of the more conservative congressional boosters of a new ABM system see this as a way of ending SALT I. In the Senate, a group of Republicans led by Sen. Pete V. Domenici (N.M.) got approval of an amendment that directs Defense Secretary Harold Brown to report on the future of ABMs by next Feb. 1.

That report, Domenici said, would aid Congress in its "reexamination of the merits of the ABM treaty" prior to the next U.S.-Soviet review, scheduled for 1982.

One major change in the new Soviet ABM system in Moscow will be introduction of a sophisticated phased-array radar complex to direct the missile intercept operations. The Soviets have already introduced phased-array radars on their early warning and perimeter defense systems.

A phased-array radar scans by electronic means and is more capable and faster than the more traditional dish-shaped radar antenna that scans the skies mechanically.

In December 1975, there was a brief publicized flurry of concern promoted by U.S. conservatives that the Soviets had breached the SALT I treaty when they introduced a phased-array ABM radar at their Siberian ICBM test site on the Kamchatka Peninsula.

The Ford administration, however, said then that it was not a violation, since such radars were permitted at test sites under the treaty.

In the fall of 1976, during the Ford-Carter presidential race, the United States and the Soviet Union signed an unpublicized agreement on how each country would go about modernizing or replacing the one ABM system each is allowed under the treaty. Details of that agreement were never made public.

Since then, however, the United States has been modifying its one large ABM radar at Grand Forks, N.D., and now the Soviets are beginning to replace at least one of their radars around Moscow.

A Pentagon analyst said the new Soviet radar was "an important qualitative change," but that there "was nothing to suggest a departure" from the limits of the SALT I treaty.

When completed, the source said, the Soviet radar is expected to be "comparable in character" to the MSR radar the United States had planned to construct for its Safeguard ABM system in the early 1970s.

The MSR system, which would have directed U.S. Sprint interceptor missiles, subsequently was canceled.

Another reason that the new Soviet radar resembles the MSR system is that the SALT I treaty limitations were drawn up based on the U.S. radar's characteristics, one defense official said last week.