U.S. experts for the first time monitored a Soviet nuclear test explosion Wednesday at this long-secret testing ground in the republic of Kazakhstan, the official Tass news agency reported.

At 100-150 kilotons, the blast had a forces several times more powerful than the bomb that devastated Hiroshima, Japan, at the end of World War II, Tass said.The Hagfors Observatory of the Swedish Defense Research Establishment said it measured the explosion at 50-150 kilotons.

Buried in a hole more than 2,000 feet underground, the nuclear explosion was not expected to have a noticeable impact at the observation site about 30 miles from the site in the vast, sparsely populated region 1,700 miles east of Moscow.

"The radiation situation in the region of the test and beyond the boundaries of the test site is normal," Tass reported after the blast at 8 a.m.

Sophisticated American sensors planted just over 10 yards from the device itself and five seismic sensors each in the United States and Soviet Union were designed to gauge the force of the blast.

The verification techniques were expected to lead the U.S. Senate to ratify two long-pending treaties limiting the size of underground blasts.