The defence ministry in a statement said that Russia's army chief Valery Gerasimov held a conversation with General Mark Milley and said that: "During the conversation... the situation related to the possible use of a 'dirty bomb' by Ukraine was continued."

The call was the latest in a string of conversations between Russian defence officials and counterparts from NATO countries, during which Moscow said, without providing evidence, that Kyiv was planning to deploy the weapon.

Gerasimov earlier in the day held a rare conversation with Britain's highest-ranking military officer, Chief of Defence Staff Admiral Tony Radakin, Moscow said, adding that the officials had also discussed a dirty bomb.

London said the call had come at the request of the Russian side.

"The Chief of the Defence Staff rejected Russia's allegations that Ukraine is planning actions to escalate the conflict, and he restated the UK's enduring support for Ukraine," the ministry of defence said in a statement.