Russia said a U.S. subcritical experiment aimed at giving data on the behavior of the materials used in nuclear warheads did not violate the Comprehensive Nuclear Test Ban Treaty.

The U.S. National Nuclear Security Administration (NNSA) said it had successfully executed a subcritical experiment May 14 at the PULSE underground laboratory at the Nevada nuclear test site.

"NNSA relies on subcritical experiments to collect valuable information to support the safety, security, reliability, and effectiveness of America's nuclear warheads, without the use of nuclear explosive testing," it said. "Consistent with the self-imposed moratorium on nuclear explosive testing that the United States has held since 1992, it did not form a self-sustaining, supercritical chain reaction."

According to the NNSA, subcritical tests use chemical high explosives to generate extreme heat and pressures that are applied to special nuclear materials in a laboratory 1,000 feet below the earth, but no self-sustaining chain reaction or criticality occurs. Computers model the data.

Russian Foreign Ministry Spokeswoman Maria Zakharova said that judging from the information provided by the United States, the experiment did not contradict Russia's understanding of a "subcritical test."

"And, accordingly, does not constitute a violation of the provisions of the Comprehensive Nuclear Test Ban Treaty (CTBT) or the U.S. moratorium on nuclear tests," Zakharova said.

Russia last year deratified the Comprehensive Nuclear Test Ban Treaty. The United States signed but never ratified the treaty.

Russia says it will not resume nuclear testing unless Washington does. Zakharova said the United States should ratify the treaty.

Russia and the United States are by far the world's biggest nuclear powers, holding about 88% of the world's total inventory of nuclear weapons, according to the Federation of American Scientists.