President Vladimir Putin said in February, without citing evidence, that some in Washington were considering breaking a moratorium on nuclear testing and that Russia should be ready to act in kind.

"... If the United States conducts tests, then we will. No one should have dangerous illusions that global strategic parity can be destroyed," Putin said.

When asked about Tracy's comments, Kremlin spokesman Dmitry Peskov played down the idea that Moscow was planning its own nuclear test.

"At the moment everyone is sticking to the moratorium. There is nothing more to say here," he told reporters.

Nuclear tensions between Russia and the United States have increased since the start of the conflict with Ukraine with Putin repeatedly warning that Russia is ready to use its nuclear arsenal if necessary to defend its "territorial integrity".

In February, Putin announced Russia was suspending its participation in the New START treaty, the last remaining nuclear arms pact with the United States, which limits the number of strategic warheads each side can deploy. 

***Reporting by Reuters; Editing by Andrew Osborn