WATCH: State Department holds news briefing as fears grow over Russian use of ‘dirty bomb’

The U.S. says it rejects the “false claim” that Ukraine is getting ready to detonate a dirty bomb.

Watch the briefing in the player above.

The Kremlin is standing by its allegation that Ukraine may be preparing to detonate a so-called dirty bomb, which would disperse radioactive material, though Ukraine, the United States, Britain and France have dismissed Moscow’s claim.

U.S. State Department spokesperson Ned Price said Monday the U.S. is “concerned that this false allegation could be used as a pretext for further Russian escalation. And we’ve made that concern very clear.”

WATCH: Ukraine warns Russia may be plotting dirty bomb attack

The Russian military says it has readied its forces for the possible use of a dirty bomb by Ukraine in a false flag attack to be blamed on Russia. That claim that has also been strongly rejected by the U.S. and its allies.

Ukraine has rejected Moscow’s claims as an attempt to distract attention from its own plans to detonate a dirty bomb, and its Western allies also dismissed the Russian claims as “transparently false.”