Soviet A-Blast Reported; U.S. Aide Says Pact on Testing Near

The Soviet Union detonated its fourth underground nuclear test in less than two months Friday, and a senior U.S. official said that a superpower agreement on limiting future test yields is “virtually” complete.

The official Tass news agency said in a two-sentence announcement that an underground test for “perfecting military technology” was carried out at the Central Asian test site.

Tass said the explosion in the Semipalatinsk region of the republic of Kazakhstan registered “within the range between 20 and 150 kilotons”--the force of 20,000 to 150,000 tons of TNT.

The test was the fourth since Moscow ended a unilateral moratorium on testing with a nuclear explosion Feb. 26. Other tests followed March 12 and April 3.

Before then, the Soviet Union had not conducted a nuclear test since July, 1985, and had sought unsuccessfully to get Washington to join a moratorium. Moscow ended its test suspension after the United States conducted its first test of 1987 on Feb. 3.

A senior American official here, speaking on condition of anonymity, said that negotiations in Moscow earlier this week during a visit by Secretary of State George P. Shultz showed the two countries to be near agreement on treaties that would limit yields in nuclear tests.

The diplomat emphasized that it was “something short of a comprehensive test ban” and said that a dispute over language was holding up an announcement. He said that Moscow wants wording referring to a total ban.

Under the proposed accord, testing below the 150-kiloton level could continue unless tighter restrictions are subsequently negotiated, the official said.

The Soviet Union had pressed for a complete ban on nuclear testing, while Washington wanted agreement on verification procedures that, it said, are necessary for ratification of two treaties negotiated in the 1970s.

In 1974, the two countries signed the Threshold Test Ban Treaty limiting tests to a maximum yield of 150 kilotons. Two years later, the Peaceful Nuclear Explosions Treaty was added, restricting the yield of tests for non-military purposes.

Although both sides have said they would abide by the treaties, Washington has never ratified them. The United States has also accused Moscow of violating the yield limit.