The Soviet Union exploded a nuclear device at the Semipalatinsk testing range of the Central Asian Republic of Kazakhstan, the official Tass news agency said Friday.

Tass said the purpose of the blast was to test research on the physics of nuclear explosions. It was the fourth test this year.The explosion had a yield of up to 20 kilotons, equivalent to 20,000 tons of TNT. Tass gave no other details of the test.

The Soviet government wants to ban all nuclear tests, but U.S. officials have resisted on the basis that they do not agree with proposed Soviet enforcement procedures.

American scientists visited the Semipalatinsk range in January to gather information that may be used in verifying U.S.-Soviet compliance with a future test ban. Similar exchanges with Soviets visiting test sites in America are planned.