The Security Council met behind closed doors to discuss Moscow's claim, first made public Sunday, that it believed Ukraine would explode a crude nuclear device in the war zone and blame Russian forces for it.

"We think it's a very serious danger," Russia's deputy UN Ambassador Dimitry Polyanskiy said after the meeting.

"Ukraine has all the reasons to do so because we know that Zelensky regime wants to avoid first of all defeat, secondly wants to involve NATO in a direct clash with Russia," he said, referring to Ukrainian President Volodymyr Zelensky.

Such a scheme is "very dangerous but will be profitable for the Zelensky regime to remain in power," Polyanskiy said.

"A dirty bomb is not a sophisticated device to be created. Actually, it is a shell with some radioactive waste. And it is very difficult to detect the activities to create these dirty bombs," he added.

James Kariuki, the British ambassador to the United Nations, rejected Russia's claim.

"We have seen and heard no new evidence during this private meeting," he said, calling the Russian claim "transparently false."

"Ukraine has been clear, it's got nothing to hide," Kariuki said.

He said that inspectors from the International Atomic Energy Agency were on the way to Ukraine after being invited by Kyiv to inspect its nuclear facilities.

"We should be clear this is pure Russian misinformation," he said.

Russian Defense Minister Sergei Shoigu made the initial "dirty bomb" claim on Sunday in calls with NATO counterparts.

Dirty bombs are crude explosives that could spew dangerous nuclear, chemical or biological materials.

The United States, France and Britain rejected Shoigu's claim in a joint statement, suggesting it was a scheme by Moscow to ratchet up the war.

"The world would see through any attempt to use this allegation as a pretext for escalation," they said.