The fourth announced underground nuclear explosion of 1986 was detonated today at the Nevada Test Site, the Department of Energy said.

The 6:59 a.m. PDT test, code-named Panamint, had an announced explosive force of less than 20 kilotons, equal to a maximum of 20,000 tons of TNT.

A department spokesman said the test was carried out without problems 1,600 feet below the desert surface at Yucca Flat, 83 miles northwest of Las Vegas.

Last August, Soviet leader Mikhail Gorbachev announced a unilateral nuclear test ban, which has been extended several times. Moscow has urged the United States to join in the moratorium, but Washington has refused, saying that U.S. nuclear testing is lagging the Soviet program.

Last Wednesday, in a speech about the Chernobyl nuclear disaster, Gorbachev extended the Soviet ban until Aug. 6.

Panamint was not announced before today's detonation. About 18 hours before the test, five antinuclear activists were arrested by security guards and charged with trespassing, a misdemeanor. The five were said to have been within 30 miles of ground zero.

Panamint was the 650th announced test since the United States began blowing up nuclear weapons at the Nevada Test Site in 1951. There have been 445 announced underground nuclear tests at the compound since the Limited Test Ban Treaty was signed in 1963. The treaty prohibits above-ground nuclear tests.

Meanwhile, an MX missile carrying a full complement of 10 dummy warheads was launched from Vandenberg Air Force Base in California on a 4,000-mile test flight to the Kwajalein Atoll in the Pacific, the Air Force said.

The dummy warheads, or reentry vehicles, are independently targetable. Today's liftoff was the first time the MX has been tested with all 10 aboard, the Air Force said.

An Air Force official in Washington who spoke on condition he not be identified said the flight was successful. It was the 12th test of the intercontinental ballistic missile in a series of 20 planned through 1987, and the third from a converted Minuteman missile silo.

By the end of this year, the Air Force plans to have 10 MX missiles in silos in southeastern Wyoming under management of Warren Air Force Base.

The Air Force has kept classified the accuracy of the new system, which is limited to 50 missiles by congressional action. But the MX was planned to be twice as accurate as the Minuteman, according to published reports, and the Air Force has announced that the MX exceeded its planned accuracy in previous test flights.