The Soviet Union is poised for a major expansion of offensive and defensive nuclear forces in the coming decade, according to a forecast prepared by the U.S. intelligence community.

According to the estimate, in a report to be released in Congress today, the Soviet Union has the capability -- if it disregards SALT II limitations -- to increase the number of nuclear warheads on its intercontinental ballistic missiles (ICBMs) from 9,000 at present to as many as 21,000 by the mid-1990s.

At the same time, Moscow has the "potential for widespread" deployment of an antiballistic missile system with radars, above-ground launchers and high acceleration missiles "well in excess" of the 1972 ABM treaty limiting such weapons, the report says.

Prepared by top officials of the Central Intelligence Agency and the National Intelligence Council, the report gives a grim assessment of Soviet potential in a world without arms control agreements, emphasizing that Moscow is well-positioned and ready for another round in the arms race despite domestic economic strain.

"We believe the Soviets are determined to prevent any erosion of the military gains the U.S.S.R. has made over the past decade," the authors said.

Defenders of past arms control agreements have long argued that the Soviet Union is in a better position to conduct a new round of the arms race than the United States, which has many fewer strategic weapons in development. This is a reason, arms controllers say, why the United States should preserve existing agreements that limit Soviet deployments. Critics of arms control, like Assistant Secretary of Defense Richard N. Perle, have expressed doubts that Moscow will build up dramatically even if arms control agreements lapse, since the Soviets already enjoy a large advantage over existing U.S. nuclear arsenals.

Although many of its findings are not new, the report discloses for the first time that Moscow is planning to deploy "large numbers" of mobile ICBMs and to have flight tests within five years of "new versions" of two ICBMs not yet deployed -- the land-based and rail-mobile SSX24 and the road-mobile SSX25.

According to the report, the Soviet ICBM force already stands a "good chance" of destroying U.S. Minuteman silos if launched, and will benefit from a "substantial increase in this damage capability" when the SSX25 and the 10-warhead SSX24 are deployed later in this decade.

Soviet SS20 intermediate-range ballistic missiles targeted at West Europe are expected to increase from 414 now to 450 by 1987, according to the report.

An "extensive modernization" in the Soviet submarine-launched ballistic missile program will result in "much better" nuclear-powered ballistic missile submarines, said the report, reflecting Moscow's efforts to close the U.S. advantage in this area.

The number of warheads on Soviet ICBMs is likely to exceed 12,000 by 1990 even if the Kremlin observes SALT II limits, according to the forecast. While the total "would not necessarily" exceed that level, the Soviets "clearly have the capability for significant further expansion" to 21,000 warheads by the mid-1990s, according to the report.

An "improved" Soviet missile defense system with 100 silo-based high acceleration missiles protecting key targets around Moscow against small-scale attacks will be completed by 1987, the report also said.