President Reagan issued a report yesterday that for the first time officially accuses the Soviet Union of "a clear violation" of the 1972 antiballistic missile (ABM) treaty by building an extensive new radar system in the central part of the country.

The report, an unclassified summary of a more detailed secret document that will be sent to Congress next week, deals with alleged Soviet violations of several arms-control agreements. It said that the phased-array radar that the United States claims is under construction at Krasnoyarsk "constitutes a violation of legal obligations" under the 1972 ABM treaty. That conclusion upgrades the administration's view in a report last year that the Krasnoyarsk radar was "almost certainly" a violation.

In addition, the report cites evidence that the Soviets have been developing and testing a variety of new ABM hardware. It concludes that "the aggregate of the Soviet Union's ABM and ABM-related actions suggest that the U.S.S.R. may be preparing an ABM defense of its national territory" in violation of the ABM treaty's prohibition of such activities.

These formal accusations about "erosion of the ABM treaty" prompted questions yesterday at a briefing for reporters about whether the administration is making the charges to justify proceeding with Reagan's Strategic Defense Initiative of research into space weaponry that can be used as antimissile shields.

The Soviets, who insisted on including space weapons in the U.S.-Soviet arms-control negotiations that begin next month, have charged that the SDI, dubbed "Star Wars," is endangering the ABM treaty.

The charges regarding the radar and associated ABM issues therefore were probably the most significant aspect of the new report, which deals with 13 "issues" where the administration suspected the Soviets had violated arms-control accords. Seven issues were dealt with in last year's report; the new report covers six additional ones.

"Today's report reaffirms our conclusions of a year ago and in some cases strengthens them," Kenneth L. Adelman, director of the Arms Control and Disarmament Agency, said. He added that the six new issues "include findings of one violation, one probable violation, one too ambiguous to call, and two cases where no violations were found."

The report includes charges that the Soviets have violated or "possibly violated" various agreements by using chemical and toxic weapons, deploying the mobile SS16 missile, testing a second land-based intercontinental missile, using new equipment associated with ABMs and not giving the West prior notification of large-scale maneuvers to be held by Warsaw Pact forces in Eastern Europe.

Many conservative Republicans in Congress have been pressing the administration to publicize suspected Soviet violations. Some have argued that the Soviet pattern of alleged noncompliance with past agreements calls into question the value of beginning the new negotiations, agreed to by Secretary of State George P. Shultz and Soviet Foreign Minister Andrei A. Gromyko in Geneva last month. The new talks will cover intercontinental strategic missiles, medium-range missiles and space weapons.

"I am frequently asked why we try to negotiate new agreements with the Soviet Union if it is violating existing ones," Adelman said. He added, though, that the administration believes that "effective and verifiable agreements can increase U.S. security," that "entering new negotiations does not in any way condone or ignore past Soviet behavior" and that "continuing to negotiate" provides "another way for trying to get the Soviets to abide by existing agreements."

But he also noted: "To be serious about arms control is to be serious about compliance."

At the press briefing following Adelman's statement, some reporters noted that much of the material in the report had already been widely publicized, and they questioned whether its release now in a formal report was prompted by the administration's desire to generate support for the SDI.

A senior administration official, who cannot be identified under the rules of the briefing, replied: "The SDI is not a violation of any existing treaty. The Krasnoyarsk radar is a clear violation of the ABM treaty."

He and other officials at the briefing noted that Congress has required that the report be made annually. One official said, "We were not hunting for violations. The Congress wishes to be informed."

The officials also suggested that it would be misleading to comb each of the 13 issues in the report for signs of how much detail the administration can muster to support its charges. They noted that in each case the report specifies how much accuracy U.S. experts attach to the available evidence, and they emphasized that in some cases the Soviets were found to be in compliance with treaty obligations.

"It is more significant to take all the findings -- whether clear or probable -- in the aggregate," one official said. "Taken in totality, it presents a troubling picture of behavior that appears to be developing in a number of areas relating to arms control."

"It is safe to say that the Soviets characteristically push each agreement to which they are a party to its limits and, in some cases, they overstep the bounds," the official added. He and the others said that is why the administration has put such emphasis on obtaining "verifiable agreements" in future negotiations with the Soviets.

Adelman noted that the United States has raised compliance issues with the Soviets repeatedly, including at last month's Geneva talks between Shultz and Gromyko. "The Soviets have not provided satisfactory explanations or undertaken corrective actions sufficient to alleviate our concerns," he said.

A phased-array radar consists of multiple units pointed in various directions to produce a widespread image. The ABM treaty bars such facilities unless located on a country's border, on the theory that inland positions could be used for no other purpose than missile defense.