"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.