"If Russia calls and says that Ukraine is allegedly preparing something, it means one thing: Russia has already prepared all this," Zelensky said in a video address on social media.

"I believe that now the world should react as harshly as possible."

If Russia had prepared "another escalating step, it must see now, preemptively and before any of its new 'dirt' that the world will not swallow it", he added.

"Even the very Russian threat of nuclear weapons -- and even more so against our country, which has given up its nuclear arsenal... is a reason for both sanctions and for even greater strengthening of support for Ukraine," said Zelensky.

Earlier Sunday, Ukraine's Foreign Minister Dmytro Kuleba denounced Moscow's claims as "absurd" and "dangerous".

"Russians often accuse others of what they plan themselves," he added.

Russia's Defence Minister Sergei Shoigu on Sunday held rare calls with NATO counterparts in which they discussed Ukraine, the Russian defence ministry said.

In calls with his British, French and Turkish counterparts, Shoigu conveyed "concerns about possible provocations by Ukraine with the use of a 'dirty bomb'".

Britain's defence ministry said in a statement that Defence Secretary Ben Wallace had "refuted" claims that Western countries sought to help Ukraine escalate the conflict.

He had also "cautioned that such allegations should not be used as a pretext for greater escalation".

Shoigu also discussed Ukraine on Sunday with Pentagon chief Lloyd Austin in their second phone call since Friday.

The Russian side did not however mention the alleged "dirty bomb" provocation in its account of that conversation.