The United States and the Soviet Union have agreed to allow unprecedented inspections of each other's nuclear test sites, and monitoring of several nuclear blasts on each other's territory next year in an attempt to agree on improvements to two treaties constraining underground nuclear explosions, U.S. officials said yesterday.

Preparations for the joint monitoring experiments are to be made during a visit by U.S. officials to the Soviet nuclear test site near Semipalatinsk, in south-central Soviet Union, beginning Jan. 7. A week or two later, Soviet specialists will visit the Nevada Test Site, according to Robert Barker, an assistant to the secretary of defense for atomic energy.

The Semipalatinsk visit is the first granted to U.S. government officials, although three U.S. members of Congress and 14 scientists associated with an independent U.S. environmental group traveled to the border of the Soviet test site in September.

Similarly, no Soviet officials have been allowed to tour the Nevada installation operated by the Department of Energy (DOE).

At least one nuclear test in each country, and perhaps two, will be monitored in the experiments, tentatively set for next April or May.

The agreement was the first substantive achievement of U.S.-Soviet discussions on nuclear testing that began in Geneva Nov. 9 after a long dispute between the Reagan administration and the Soviet Union over continuing underground nuclear tests.

The accord was reached as the House Permanent Select Committee on Intelligence released a lengthy criticism of the administration's position on nuclear tests and its approach to the two treaties in question, the Threshold Test Ban Treaty of 1974 and the Peaceful Nuclear Explosions Treaty of 1976.

The panel said the United States has equipment in hand to verify Soviet compliance with a future test limit of only 15-20 kilotons, less than one-sixth the size of blasts now allowed by the treaties.

The administration asserts that nuclear tests are essential to maintaining and improving the U.S. nuclear weapons arsenal. The Soviets back a ban on all nuclear tests, a position that is aimed in part at limiting U.S. research on weapons for ballistic missile defense.

Meanwhile, Soviet officials completed a two-day visit yesterday to the U.S. Army chemical weapons storage site near Tooele, Utah. The Reagan administration, reciprocating for a U.S. visit to a Soviet chemical weapons facility last month, invited the Soviets to witness a simulated destruction of aging chemical stockpiles.

The inspection was intended to further multilateral negotiations in Geneva toward a worldwide elimination of chemical weapons.

The two sides recently agreed that the talks will deal first with new measures to verify compliance with the 1974 and 1976 treaties, which limit nuclear tests to 150 kilotons, about 150,000 tons of TNT, as a way to stop development of unusually powerful new nuclear weapons. Barker, who spoke in Geneva at the end of the first round of negotiations, said the Soviets demanded the joint monitoring experiments last April to determine the efficacy of a U.S.-backed verification measure known as CORRTEX, for Continuous Reflectrometry for Radius versus Time Experiment.

The experiments will involve placing an electrical cable at the top of the hole in which U.S. and Soviet nuclear weapons are buried before being exploded, as well as a special hole nearby. A separate monitoring device determines the force of the blast by measuring the rate at which the cable is crushed by resulting shock waves.

U.S. officials claim the CORRTEX technique will substantially improve U.S. capability to monitor Soviet compliance with the two testing treaties, although this claim was disputed yesterday by the intelligence committee.

"It is debatable whether the difference between CORRTEX-derived yield measurements and those derived by {existing methods} will differ to any militarily significant degree," the committee said after hearing testimony by government scientists and independent experts.

The administration has argued that improved verification measures are needed because the Soviets have "likely" violated the testing treaties, but this was also disputed by the committee. "The preponderance of testimony received did not support executive branch allegations" of the violations, the report said.

The Soviets have told the administration they are willing to go along with its demand for verification improvements, but favor an alternative technique for measuring seismic waves created by the blasts from a slightly greater distance. U.S. officials in Geneva said the Soviets plan to demonstrate the seismic technique during the two tests next year.

U.S. officials said that once the Soviets agree that incorporate CORRTEX into the treaties, the administration will seek Senate ratification and begin negotiations on further testing limitations, as the Soviets have sought.

The two superpowers sharply disagree about what these limitations should be and when they should take effect, however.