Western officials were trying to decipher Moscow’s motives on Monday after jointly rejecting a Russian allegation that Kyiv is preparing to deploy a so-called dirty bomb in Ukraine.

The U.S., France and U.K. warned Russia that “the world would see through any attempt to use this allegation as a pretext for escalation” of the war, in a statement issued late Sunday night.

The pushback follows an unusual flurry of phone calls on Sunday by Russia’s defense minister, Sergei Shoigu, to his U.S., French, U.K. and Turkish counterparts. He told them that the war in Ukraine was moving toward a more dangerous phase and that Kyiv might soon deploy a dirty bomb, which combines conventional explosives with radioactive materials and would contaminate a swath of territory without immediately killing a large number of people.

Western officials say, however, that they can’t be too dismissive of Russia’s dirty-bomb claims because Moscow could be signaling that it is preparing to use such weapons itself in a false-flag operation and then use it as an excuse for deploying a nuclear weapon.