The United States, Britain and France on Sunday jointly dismissed Russian claims that Ukraine is preparing to use a dirty bomb and warned Moscow against using any pretext for escalating the conflict.

Russian Defense Minister Sergei Shoigu made the allegation about a possible dirty bomb attack in a round of telephone conversations with Western defense chiefs earlier Sunday.

"Our countries made clear that we all reject Russia's transparently false allegations that Ukraine is preparing to use a dirty bomb on its own territory," the US State Department said in a joint statement with the British and French governments.