US Defense Secretary Robert Gates expressed concern Wednesday that Russia might not be accounting for all nuclear weapons produced during the Soviet era.

"I have fairly high confidence that no strategic or modern tactical nuclear weapons have leaked" beyond Russian borders, Gates said at the Carnegie Endowment for International Peace.

"What worries me are the tens of thousands of old nuclear mines, nuclear artillery shells and so on, because the reality is the Russians themselves probably don’t have any idea how many of those they have or, potentially, where they are,” AP quoted him as saying.

The Russian Foreign Ministry dismissed his remarks in a statement.

"Despite all the difficulties our country faced in the beginning of the 1990s, standards of security and physical protection of Russian nuclear arsenals remained high," the statement said. "There have been no ‘leaks’ of nuclear weapons"