Senior Russian military leaders have discussed when and how they would use tactical nuclear weapons in Ukraine, multiple US officials have claimed, the media reported.

They did not talk about using the weapons with Russian President Vladimir Putin -- but the conversations have heightened concern about the prospect of a nuclear Armageddon, Daily Mail reported.

US government officials learned about the discussions in mid-October, as Moscow's nuclear rhetoric intensified, The New York Times reported.

It comes after Putin joked about the prospect of a nuclear war last week.

The Kremlin leader was asked to reassure an audience at the Valdai Discussion Club think-tank that the world is not on the verge of nuclear annihilation -- and chose to respond with a long pause.

When host Fyodor Lukyanov pointed out his silence was 'alarming', a smirking Putin responded: "I did that on purpose so you would be on your guard. The effect has been achieved."

John F. Kirby, a National Security Council official, told the New York Times: "We've been clear from the outset that Russia's comments about the potential use of nuclear weapons are deeply concerning, and we take them seriously.

"We continue to monitor this as best we can, and we see no indications that Russia is making preparations for such use."

Russia's nuclear stockpile, the largest in the world, consists of 'tactical', lower-yield bombs and strategic weapons that can annihilate cities and population centers.

Its tactical nukes, with a yield of between ten and 100 kilotons, are designed for use on the battlefield in contested territory.

In comparison, the atomic bomb dropped on Hiroshima in 1945 was approximately 18 kilotons, Daily Mail reported.

(KB/IANS)