The Reagan administration yesterday brushed aside a new Soviet proposal for halting underground nuclear tests, but officials indicated that internal discussions are under way about a new U.S. offer on nuclear test monitoring.

President Reagan revealed Wednesday in a letter to Sen. Robert J. Dole (R-Kan.) that he had proposed to Soviet leader Mikhail Gorbachev in December that the two nations convene a meeting of nuclear testing experts in February "to discuss our respective verification approaches and to address initial tangible steps to resolve this issue."

Reagan said the Soviets did not respond to that offer. Gorbachev yesterday offered to extend the Soviets' self-imposed moratorium on testing indefinitely until the United States detonates another nuclear bomb.

Other administration officials said Reagan's proposal in December had been prompted by a Dec. 5 letter from Gorbachev asking that the United States join the Soviet moratorium. The Soviets have expressed willingness to permit some on-site inspections to verify a ban on underground nuclear tests; the United States is seeking on-site monitoring to verify that continued tests do not exceed certain limits.

A State Department official said "a lot of work has been taking place" in U.S. agencies concerned with verification of nuclear tests but that no new conclusions have yet been presented to the Soviets.

Reagan brought up the testing issue Wednesday in an interview with The Baltimore Sun, saying that he offered to permit Soviet experts to "come here with whatever equipment they wanted to bring and be witness to one of our underground tests." He noted that the United States believes that in the past the Soviets exceeded the limit of 150 kilotons established by the 1974 Threshold Test Ban Treaty. The United States has never ratified the treaty, but both sides have said they are respecting its provisions.

White House spokesman Larry Speakes said yesterday that the United States "places a high priority" on enhancing means of verifying the size of underground nuclear explosions as a step toward U.S. ratification of the Threshold Test Ban Treaty and the 1976 Peaceful Nuclear Explosions Treaty.

Speakes, responding to a new Soviet extension of its self-imposed moratorium on nuclear tests, told reporters yesterday that "it is necessary for us to conduct tests and our allies to conduct tests in the near future" because nuclear weapons are important to the U.S. deterrent force and because the Soviets have made gains in this area.

Gorbachev has said he looks to progress on the testing issue, along with movement toward an accord on intermediate-range nuclear forces (INF) in Europe, as moves that would justify scheduling another summit meeting with Reagan this summer.

Moscow imposed a unilateral halt to its testing last August and has continued it through several installments ever since. The last announced U.S. weapons test was last Dec. 28. A new test series at the Nevada test site would normally be expected to begin by mid-March.

A spokesman at the test site said, "We are preparing for a test . . . . We have a test program we are following." But he said test dates are classified and that no advance announcements are made except when the tremors are large enough to cause concern in nearby towns.

Last week, the State Department confirmed reports that Soviet diplomats had "given faint hints that they might consider some improved verification arrangements" on the Threshold Test Ban Treaty. But a spokesman added, "The Soviet government has made no proposal in this regard."