The first U.S. underground nuclear weapons test of 1986 -- believed to be an early warhead design for the new Midgetman mobile intercontinental missile -- was detonated as planned yesterday at the Nevada Test Site, provoking immediate criticism from the Soviet Union.

Within minutes after the shot was announced, Tass news service in Moscow described it as "the Reagan administration's new militaristic action," but gave no indication whether Soviet leader Mikhail Gorbachev will use the test as a reason to end his self-imposed moratorium on Soviet testing.

The Soviet leader said on March 13 that the moratorium, which began last August and was to end March 31, would remain in effect until the next U.S. test. However, Soviet officials in Washington subsequently informed members of Congress that the first U.S. test after March 31 would trigger the resumption of Soviet testing, one congressional source said.

A State Department spokesman said yesterday that U.S. nuclear testing would continue "to insure the credibility and effectiveness of the U.S. nuclear deterrent."

"Under existing conditions," the spokesman added, "neither a moratorium nor a comprehensive test ban would enhance the cause of security, stability or peace."

During the Soviet eight-month moratorium, the United States exploded seven announced underground nuclear tests, according to informed sources. The first took place three weeks after Gorbachev announced his initiative.

"Gorbachev has got to go ahead," Steven Meyer, a specialist in Soviet military weapons systems, said yesterday. Noting that Gorbachev had already extended the moratorium once while U.S. tests continued, Meyer said, "For him to hold on much longer would require a major movement on the part of the United States."

Meyer added that he expects the Soviets to test "at an increased rate" in the next year both to make up ground lost during the moratorium and to accelerate development of new air- and missile-defense weapons.

In permitting the test to take place, the White House ignored last-minute pleas for a delay from more than 60 members of Congress. The only pause in yesterday's test was a 15-minute delay in the shot caused by unexpected winds.

Administration officials have refused to say what kind of weapon was involved in the test, code-named Glencoe and detonated by Los Alamos National Laboratory 2,000 feet below the Nevada desert north of Las Vegas. Privately, however, they ruled out any connection with the president's Strategic Defense Initiative, the so-called Star Wars research program.

Sources involved in the nuclear program said the device exploded was related to the new Midgetman warhead, which is one of three nuclear weapons now in the early stages of development.

In testimony before Congress in opposition to a ban on testing, Reagan administration officials have argued that they need to continue explosions to design the Midgetman warhead.

Pentagon officials argued that a new warhead was needed, rather than using one designed for a silo-based missile, because it had to withstand different stresses caused by movement of the mobile missile.

In their plea to Reagan to halt yesterday's test, members of Congress said that Midgetman did not need a new warhead. "On the contrary," the legislators wrote, "the Air Force plans to use the present MX warhead, and there is no need for a new device."

The United States conducted 17 tests last year, a rate that is expected to be continued this year, according to informed sources. The Soviets tested seven times in 1985 before the moratorium. The year before, they held 16 reported weapons tests, according to a study prepared by the National Resources Defense Council, a nonprofit research group studying nuclear matters.

The previous U.S. nuclear test, held Dec. 28 and codenamed Goldstone, was associated with an X-ray laser concept that is part of Star Wars.

On March 15, Reagan wrote to Gorbachev and invited Soviet scientists to observe a nuclear test the third week in April. The president, while opposing an end to tests, has been pressing for better verification methods as the price of U.S. ratification of the 1974 threshold test ban treaty. That agreement, signed during the Nixon administration but never ratified, limits both nations to tests of 150 kilotons or less.

Reagan has charged that some Soviet tests have exceeded that limit; he wants both nations to use on-site monitoring devices to verify the size of explosions. He has offered to show the Soviets such a device at the April test in hopes that the Soviet Union would allow U.S. technicians to use it when they resume testing.

Moscow appeared to reject the Reagan offer, as it has with similar ones in the past. But U.S. officials last week said Gorbachev has never formally replied.