WASHINGTON (AP) _ Soviet leader Mikhail Gorbachev, in a letter to President Reagan, has offered to let American inspectors visit Soviet underground nuclear test sites, a U.S. official said Thursday.

But Gorbachev made the proposal conditional on U.S. acceptance of a moratorium on further tests. The White House, while not referring to the letter, rejected such a link - at least for now.

In the letter, Gorbachev also urged Reagan to approve a resumption of U.S.-Soviet negotiations next month on a treaty to ban all such tests, the official told The Associated Press.

The offer to permit U.S. inspectors at Soviet test sites to clear up ''ambiguities'' has the potential of giving the lagging arms control process a major push forward in the aftermath of the Reagan-Gorbachev summit meeting.

The official, who demanded anonymity, said the offer was conditional on Reagan accepting a moratorium on further tests. A Soviet self-imposed ban expires Jan. 1.

White House spokesman Larry Speakes said ''we will be glad to continue to talk to them about on-site inspection, but as far as a moratorium ... we would not agree to it at this time.''

A U.S. nuclear test due to be conducted Thursday was postponed, but there was no indication the action was related to the Soviet proposal.

The official said he did not know what the president's response to Gorbachev would be. Charles Redman, a State Department spokesman, said the two sides had engaged in ''confidential exchanges'' but refused to say anything regarding the letter.

It was learned, meanwhile, that Soviet ambassador Anatoly F. Dobryin called on Secretary of State George P. Shultz on Wednesday night soon after Shultz returned from a 10-day trip to Europe.

In offering to open Soviet testing areas to U.S. observers, Gorbachev suggested Reagan agree to accept an offer by the leaders of six countries to permit the installation of seismic and other monitoring devices on their own territories.

The six are Argentina, Greece, India, Mexico, Sweden and Tanzania. The idea would be to expand the potential for verification of U.S. and Soviet compliance with a testing freeze.

If there were still ''ambiguities'' afterward, Gorbachev said, American inspectors could go to Soviet test sites to see for themselves. He asked for a reciprocal right to send Soviet inspectors here.

Reagan in July said he would be willing to allow Soviet observers to witness a U.S. nuclear explosion at a Nevada test site. ''They could come tomorrow, if they like,'' Speakes said.

The principle of on-site inspection was established by the superpowers a decade ago when they agreed on a treaty to limit underground nuclear explosions for such nonmilitary purposes as diverting rivers to 150 kilotons.

The provision was never put into effect, and the Soviets traditionally are reluctant to permit American monitors on Soviet soil.

A treaty in 1974 limited underground weapons tests to no more than 150 kilotons.

Neither accord was ratified by the Senate. In a report to Congress last February, Reagan said the Soviets ''likely'' had violated terms of the treaty.

In Moscow, meanwhile, the official Communist Party daily Pravda said in an editorial that the Soviet Union would consent to ''certain measures of on-site verification'' if the United States agreed to a comprehensive ban on nuclear testing.

But Pravda reiterated that the five-month-old unilateral Soviet test moratorium would be extended only if the United States joins. U.S. officials have said the freeze is one-sided and that the Soviets had already concluded their testing program.

The director of the U.S. Arms Control and Disarmament Agency, Kenneth Adelman, said he hoped the editorial signaled a Soviet willingess to accept Reagan's invitation to Soviet experts to visit U.S. test tests.

Adelman, in an interview, said he welcomed ''the Soviet focus on verification,'' which the U.S. official called ''the big stumbling block in arms control.''

Meanwhile, 46 senators and 106 members of the House urged Reagan in separate letters to resume test ban negotiations as ''an excellent way to build on the climate of cooperation that persists in the aftermath'' of the summit.

''We believe that the resumption of such talks will demonstrate to the world that both you and Mr. Gorbachev are determined to take concrete steps to further reduce superpower tension and foster a more stable international environment,'' said the senators' letter, drafted and sponsored by Sens. Charles McC. Mathias, R-Md., and Edward M. Kennedy, D-Mass.

The House letter, drafted by Reps. Berkley Bedell, D-Iowa, and Jim Leach, R-Iowa, said that in light of the approaching end of the Soviet moratorium, ''unless the United States acts now another important opportunity to end nuclear testing will come to an end.''