Russia may be violating a ban on the testing of low-yield nuclear weapons capabilities at a site in the Arctic, a top US intelligence official said.
Lt Gen Robert Ashley, the director of the US Defense Intelligence Agency, said Moscow was "probably not adhering to" the rules of a recognised treaty.
He was referring to the Comprehensive Test Ban Treaty (CTBT), a multilateral agreement prohibiting nuclear testing.
Russia, which ratified the treaty in 2008, says it complies with the CTBT.
The US has signed but has not yet ratified the treaty.
"Our understanding of nuclear weapon development leads us to believe that Russia's testing activities would help it improve its nuclear weapons capabilities," Lt Gen Ashley said on Wednesday.
He added that the US expected Russia, which he said was likely testing weapons in the Novaya Zemlya islands, to increase its nuclear arsenal "significantly" over the next decade.
But analysts received the statement with scepticism. The Comprehensive Nuclear Test Ban Treaty Organization (CTBTO) said in a statement that it had not detected any unusual activity.
"The CTBTO has full confidence in the ability of the IMS [its monitoring system] to detect nuclear test explosions,", the organisation said in a statement.
The CTBT, which bans nuclear weapons testing anywhere in the world, was adopted by the UN General Assembly in 1996. It sets out nuclear disarmament as a principle but diplomatically avoids the politics of the issue.