The White House has said it was increasingly concerned over Moscow's talk of using a nuclear weapon in Ukraine, after a media report said top Russian military officials had discussed how and when to use such a weapon.

"We have grown increasingly concerned about  the potential as these months have gone on," said White House national security spokesman John Kirby.

Kirby did not confirm a New York Times report that said high-level Russian military officials recently discussed when and how they might use tactical nuclear weapons on the battlefield.

The report, which cited unnamed US officials, said Russian President Vladimir Putin did not take part in the discussions, and there was no indication that the Russian military had decided to deploy the weapons.

But Kirby said any comments on the use of nuclear weapons by Russia are "deeply concerning," and said the United States takes them seriously.

He pointed to recent Putin comments talking about nuclear weapons and referencing the bombs US forces dropped on the Japanese cities Hiroshima and Nagasaki near the end of World War II.

"We take note of that," Kirby said. "It increasingly is unsettling in terms of the degree to which he feels he has to continue to stretch to prosecute this war," he said.

At the same time, Kirby reiterated, Washington sees no indications that Russia is making preparations to use nuclear weapons, adding that US intelligence does not necessarily see or know everything.

The United States has been warning Moscow for weeks over public comments from top Russian officials that they could use nuclear weapons in Ukraine in certain cases, particularly if they felt there was a threat to Russian territorial integrity.

The most recent threat came from former Russian president and senior security council official Dmitry Medvedev.

Medvedev said on Tuesday that Ukraine's objective to reclaim all its territories occupied by Russia, which include the Donbas region and Crimea, would be a "threat to the existence of our state."

That, Medvedev said, would be "a direct reason" to invoke nuclear deterrence.

However, early Wednesday Putin's spokesman Dmitry Peskov said that Western media was "deliberately pumping up the topic of the use of nuclear weapons."

Moscow does "not have the slightest intention to take part in this," he said, calling the Times report "very irresponsible."

In September, Jake Sullivan, President Joe Biden's national security advisor, said that the United States has warned Russia at "very high levels" of "catastrophic consequences" for using nuclear arms.

EU foreign policy chief Josep Borrell warned on October 13 that Russian forces would be "annihilated" by the West if Putin uses nuclear weapons against Ukraine.

Russian 'top priority'

Russia said Wednesday the world's "top priority" should be to avoid a clash of nuclear powers, stressing it could lead to "catastrophic consequences".

"We are firmly convinced that in the current difficult and turbulent situation -- a consequence of irresponsible and shameless actions aimed at undermining our national security -- the top priority is to prevent any military clash of nuclear powers," said the foreign ministry.

Moscow called on other nuclear powers to "abandon dangerous attempts to infringe on each other's vital interests".

The foreign ministry's statement came amid rising fears of nuclear use in the Ukraine conflict, which has dragged on into its ninth month.

"Russia is strictly and consistently guided by the tenet that a nuclear war cannot be won and must never be fought," the statement read.

It said Moscow's nuclear doctrine is "purely defensive in nature", only allowing the Kremlin to use such weapons in the event of nuclear aggression or "when the very existence of our state is threatened."

Russia has repeatedly suggested that Ukrainian territories it claims to have annexed are protected by its nuclear doctrine.

The statement called on the world's other nuclear powers -- the United States, Britain, France and China -- to "work together to solve this priority task".