If the United States halts modernization of radar sites in Greenland and Britain, the Soviet Union has offered to halt construction of a giant new radar in central Siberia that President Reagan has called an arms treaty violation, according to U.S. officials.

State Department spokesman Bernard Kalb called the proposal "inequitable and an unacceptable precedent" yesterday because it equated a Soviet development prohibited by the 1972 Antiballistic Missile (ABM) Treaty with a U.S. modernization the treaty allows.

However, administration spokesmen stopped short of rejecting the Soviet offer, which Kalb said is still "an item certainly on the agenda." One U.S. official added that it was the first time Moscow appeared to accept arguments that its facility could be a violation.

Until this offer was made to U.S. arms negotiators in Geneva several weeks ago, the Soviets had rejected U.S. protests that the Siberian radar violated the treaty. Instead, Moscow said the site, at Abalakovo near Krasnoyarsk, was a permitted space-tracking facility.

The treaty forbids putting such a radar anywhere other than along the borders of the Soviet Union. The pact also requires the radar to face directly outward so it cannot be used to guide antiballistic missile interceptors.

Moscow, for its part, has several times called the upgrading of the U.S. radars in Greenland and Britain a treaty violation.

One U.S. official said the American facilities are so vital to the ballistic missile early warning network that "the Soviets must know they made an offer we could not accept."

He added that many government experts view the proposal as a move "to appease public opinion in the United States," where even critics of the administration's arms control approach have agreed with the Reagan finding that the Siberian radar is a violation.

Arms control experts inside and outside the government said the Soviet initiative was another indication that Soviet leader Mikhail Gorbachev may be ready to make offers on a wide range of contentious issues in the three weeks leading up to his summit meeting with President Reagan next month.

John B. Rhinelander, legal adviser to the U.S. delegation that negotiated the ABM Treaty, called the Soviet offer "a very favorable sign if it leads to dismantling" of what he said is a clearly illegal Siberian facility. Rhinelander also said, however, that the upgrading of the two U.S. radars "does raise serious questions" about U.S. interpretation of the treaty, which he said does not clearly permit such a modernization.

Under the Soviet offer, which was first disclosed three days ago in the London Sunday Times, Soviet ambassador Yuli Kvitsinsky told U.S. negotiators in informal discussions that Moscow was willing to trade the Siberian radar in Krasnoyarsk for a halt to the modernization of U.S. radars in Thule, Greenland, and Fylingdales Moor, England.

Thule, Fylingdales Moor and another large radar at Clear, Alaska, make up the 20-year-old American ballistic missile early warning system (BMEWS) that would detect Soviet missiles launched over the North Pole toward the United States.

When the 1972 ABM Treaty was signed, these three sites were specifically agreed to by the Soviets.

A plan drafted in the 1970s to modernize the radars was financed in 1981 and 1982. The four football-sized radars at Thule will be replaced by one large "phased-array" facility which will become operational in October 1986. A former government arms control expert said yesterday, "There is no way that Thule could be used other than for early warning."

The upgraded radar at Fylingdales Moor in Britain, which will vastly increase early-warning capabilties, also may provide battle-management capabilities for some future European antimissile system, according to government sources. Soviet protests that the upgrade is a potential treaty violation have led to British delays in permitting the project to proceed.

Initial Defense Department funding for Fylingdales Moor is included in next year's Pentagon budget. The facility is scheduled to become operational in 1990, according to presentations to Congress.