The Soviet Union exploded a small nuclear device Tuesday at its main underground testing site in Semipalatinsk, in the Kazakhstan region, the official Tass news agency said.

Tass said the device, with an explosive force of less than 20 kilotons of TNT, was exploded for the purpose of "research into the physics of a nuclear explosion" - an explanation that stopped short of clarifying whether it was a military or civilian test.