Moscow has repeatedly warned in recent days that Ukraine could employ a dirty bomb -- which spreads nuclear, chemical or biological material via conventional explosives -- sparking fears that Russia could use such a device and blame Kyiv.

"The Ukrainians are not building a dirty bomb, nor do we have indications that the Russians have made a decision to employ nuclear, chem, bio" weapons, the military official told journalists on condition of anonymity.

Asked if that assessment included dirty bombs, the official replied, "Yes."

Russian Defense Minister Sergei Shoigu has had a series of recent phone calls with counterparts from several NATO countries in which he said Kyiv was planning to use a dirty bomb.

On Monday, Russia's defense ministry said army chief Valery Gerasimov made the same allegation in a conversation with US General Mark Milley.

"During the conversation... the situation related to the possible use of a 'dirty bomb' by Ukraine was continued," the ministry said in a statement.

The US military also confirmed the call, saying the two discussed "several security-related issues of concern and agreed to keep the lines of communication open."

The United States, Britain and France on Sunday jointly dismissed Russia's dirty bomb claims, and Ukrainian President Volodymyr Zelensky rejected the accusations as well.

"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.

Russia has suffered a series of military defeats in southern and eastern Ukraine, with observers of the conflict and Kyiv saying the Kremlin was becoming increasingly desperate.