According to multiple senior U.S. officials, senior Russian military leaders have recently had conversations about when and how Moscow might use tactical nuclear weapons in Ukraine, raising concerns in Washington and allied capitals, reports The New York Times.

Russian President Putin allegedly did not participate in these conversations, which took place against the background of the Kremlin’s increased nuclear rhetoric and Russian failures on the battlefield. But the fact that senior Russian military leaders participated in such discussions worried the administration of US President Joe Biden.

Still, U.S. officials said they had seen no evidence that the Russians were moving nuclear weapons into locations or taking other tactical steps to prepare for a strike. Intelligence about these conversations was shared within the US government in mid-October.

American officials did not describe the scenarios considered by the Russian military leadership for the use of nuclear weapons. However, William Burns, director of the CIA, has previously said that Putin’s “potential desperation” over defeats in Ukraine and failures in the war could push Russia to use nuclear weapons.

John F. Kirby, a spokesman for the US National Security Council, declined to comment on “the details of this report.”

“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,” Mr. Kirby said. “We continue to monitor this as best we can, and we see no indications that Russia is making preparations for such use.”

According to Pentagon estimates, Russia has up to 2,000 tactical nuclear weapons, which are intended for use on the battlefield to suppress the enemy’s armed forces. No tactical nuclear weapon has ever been used in combat, but it can be deployed by any means, including rockets or artillery shells.

Tactical nuclear weapons have less power and are designed for use at shorter distances than ICBM warheads.

Military experts say that the use of nuclear weapons – for the first time in 75 years – would fundamentally change the shape of war. Although the resulting destruction will depend on many factors, including the size of the weapon and the direction of the winds, even a small nuclear explosion could kill thousands of people and render parts of Ukraine uninhabitable.

While the risk of further escalation remains alarmingly high, Biden administration officials and US allies also say phone calls between Western and Russian counterparts late last month have helped ease some nuclear tensions. Putin’s speech last Thursday, in which he denied that Moscow was preparing to use nuclear weapons in Ukraine, further lowered the temperature, according to some officials.

In Washington, administration officials say they still don’t believe Putin plans to use tactical nuclear weapons or even a “dirty bomb.”