The United States is demanding that the giant Soviet early-warning radar being constructed near Krasnoyarsk in central Siberia be modified or torn down before any new understanding is reached on the 1972 Antiballistic Missile (ABM) Treaty, according to U.S. sources.

Even some Soviet officials admit privately that the radar complex violates the ABM pact.

During last week's summit discussions in the arms control working group, the U.S. delegation made clear that there would have to be action on the radar facility before any step could be taken on Moscow's desire that the superpowers agree to continue to adhere to the ABM Treaty for seven to 10 years, administration sources said. For three years President Reagan has labeled Krasnoyarsk a clear violation of the treaty.

Yesterday, during an interview on ABC's "This Week With David Brinkley," Reagan's national security adviser, Lt. Gen. Colin L. Powell, said, "Our position is that Krasnoyarsk must come down."

"The Soviets are reflecting on it. They haven't agreed with us yet, but we're looking for a way to solve this problem," he added.

In private conversations last week, one key Soviet official said he believed that destruction of Krasnoyarsk would be a dramatic gesture of the new U.S.-Soviet relationship. U.S. technical experts added that they believed a satellite system could replace Krasnoyarsk's early-warning capability within several years.

On Capitol Hill, some Senate critics of the newly signed Intermediate-Range Nuclear Forces (INF) Treaty have talked about amending the agreement during the ratification process to require that Krasnoyarsk be destroyed before the pact could go into effect, according to congressional sources.

Meanwhile, other Soviet officials, including some in the military, have recently described how in the early 1970s the Soviet Defense Ministry decided to go ahead with the facility knowing it violated provisions of the just-signed ABM Treaty.

"The blame is being placed on Dmitri Ustinov," the former Soviet defense minister and influential Politburo member who died in December 1984, one American expert said yesterday.

Under the ABM Treaty, so-called phased array radars that were to be used for early warning of a missile attack were to be placed along the periphery of the United States and the Soviet Union. They also were to look outward so they could not be used to manage a nationwide ballistic missile defense system, which was prohibited by the treaty.

The Soviet network of nine modern, electronic phased-array radars was initially planned by the Soviet Defense Ministry during the late 1960s, Soviet and U.S. sources said.

According to several sources, Soviet defense officials placed eight of the radar complexes at sites near the border of the country, but chose the Krasnoyarsk site to save money. Had it been placed closer to the periphery -- as required by the treaty -- more than one facility would have had to be built and the cost would have been "10 to 20 times larger," according to these sources.

Politburo approval of construction of the entire network took place in the early 1970s, about the time the treaty was being signed.

In a version of Soviet events that several U.S. officials question, a Soviet official said that Ustinov did not tell the Politburo that Krasnoyarsk potentially violated the treaty, but said that then-foreign minister and Politburo member Andrei Gromyko and the Politburo legal staff must have known.

The Defense Ministry expected the violation to be discovered by U.S. photo intelligence, sources said, but counted on keeping Washington protests bottled up quietly for several years in the joint U.S.-Soviet committee set up by the ABM Treaty. All discussions of potential treaty violations at this Standing Consultative Committee (SCC) were by agreement to be kept secret, according to both Soviet and U.S. sources.

Meanwhile, the Soviets planned to try to trade continued construction for allowing what they would charge were similar U.S. violations. At first they brought up the so-called Pave Paws radars being built in Massachusetts and California, and later the U.S. upgrading to large electronic radars of units allowed in Fylingdales, England, and Thule, Greenland.

When Reagan in March 1983 announced his space-based antimissile program, the Strategic Defense Initiative (SDI), Soviet Defense Ministry officials used its potential to break the ABM Treaty as justification for their earlier decision to build Krasnoyarsk, Soviet sources said.

The Pentagon discovered the construction at Krasnoyarsk in July 1983, more than 18 months after it began, according to U.S. sources. "About a month or so later," according to a U.S. source, the issue of the radar complex was raised with the Soviets at the SCC. At that time, the Soviets described it as a "space-tracking" facility that was permitted under the ABM Treaty.

Less than a year later, however, the Reagan administration publicly charged Krasnoyarsk was an ABM Treaty violation and made it the centerpiece of its attack on Moscow for past treaty violations.

Recognizing the depth of U.S. feelings about Krasnoyarsk, Soviet leader Mikhail Gorbachev began to take steps to meet the problem. He permitted three U.S. congressmen -- Reps. Bob Carr (D-Mich.), Thomas J. Downey (D-N.Y.) and Jim Moody (D-Wis.) -- along with some American scientists to visit the site in October. They returned to the United States and declared that the facility did violate the treaty.

Last month, Gorbachev announced a one-year moratorium on its construction, but at the same time said he would "expect" a comparable U.S. delay at Fylingdales.

Meanwhile, some U.S. officials want Krasnoyarsk destroyed while others want to permit it to be modified. One plan is to build a high wall around it so that the radar could only look up into space and not out at the horizon in violation of ABM treaty provisions.

The Soviets also are unsure what to do. A Soviet military spokesman told reporters it should be completed, while one of Moscow's leading arms control experts said privately that modification of the facility would cost too much for the benefits. "Space tracking," he said, "could be done better elsewhere," indicating he would be prepared to see the already constructed buildings destroyed.