Washington, Mar 23 (IANS): The Pentagon has condemned a Kremlin spokesman's refusal to rule out the use of nuclear weapons during the Ukraine conflict, BBC reported.

Dmitry Peskov told CNN International that such arms could be used if Russia faced an "existential threat". Russia has the world's largest stockpile of nuclear warheads.

Department of Defence spokesman John Kirby said Moscow's nuclear remarks were "dangerous".

"It's not the way a responsible nuclear power should act," he told reporters.

But Kirby added that Pentagon officials "haven't seen anything that would lead us to conclude that we need to change our strategic deterrent posture."

"We monitor this as best we can every day," he added.

Former US Defence Secretary Leon Panetta also criticised Peskov's comments.

"I don't see how you can see it in any other way but as dangerous when Russia is looking for a possible excuse for the use of low-yield nuclear weapons," Panetta told CNN.

"And basing it frankly on a very false premise that somehow Russia is being threatened. I think that presents a real concern that Russia at least is considering that possibility.

"I don't know that it's going to happen. I still think that Putin has to worry about how the US would respond and he has to worry about his own survival."

Putin put his country's nuclear forces on special alert on February 27, but Western defence officials said afterwards they had not seen any significant sign of Russia's strategic bombers, missiles and submarines being mobilised.