Nevertheless, it drew coordinated public pushback from the U.S. and its allies.

In a joint statement early Monday, the U.S., Britain and France rejected Shoigu’s accusations and “any pretext for escalation by Russia.” Separately, Secretary of State Antony Blinken called the allegations “transparently false.”

Later on Monday, a senior U.S. defense official reiterated the growing chorus of Biden administration officials denying Russian claims.

“These allegations are false,” the official said. “The Ukrainians are not building a dirty bomb.” 

Kremlin spokesman Dmitry Peskov countered those statements Monday, saying the threat of a “dirty bomb” was “obvious,” irrespective of the doubts of Kyiv’s allies. In a lengthy update Monday, the Russian Defense Ministry said Ukraine had the scientific, technological and industrial capability to create a “dirty bomb” to further discredit Russia globally.

The senior American defense official said there was no indication the Russians have decided to use nuclear, chemical or biological weapons in Ukraine. 

Ukrainian President Volodymyr Zelenskyy also denied Shoigu’s claims in a video address Sunday, adding that Shoigu’s “telephone carousel” with Kyiv’s allies was not very convincing.

“If Russia calls and says that Ukraine is allegedly preparing something, it means one thing: Russia has already prepared it,” he said.

Kyiv on Monday said Russia was engaging in “nuclear blackmail,” adding that it was open to monitoring missions.

Oct. 24, 202202:04

The Kremlin has an established pattern of warning about so-called “false flag” operations by Ukraine and its allies.

The U.S. said in January that Russia was engaging in such a scheme in eastern Ukraine to try and justify a full-scale invasion. Moscow did just that in the buildup to its Feb. 24 assault, and has subsequently accused Ukraine and its allies of planning biological and nuclear provocations in order to blame Russia.

Moscow’s latest accusations offered few specifics. But broadly, a “dirty bomb” can be defined as a conventional explosive with added radiological material, according to the Washington-based Center for Arms Control and Non-Proliferation. Its deployment could threaten tens of thousands of people and contaminate the affected area for up to 50 years, a Russian nuclear official was quoted as saying by the state news agency Tass on Monday. NBC News could not verify the claims.

Some analysts picked up on Shoigu’s rush of calls as a potentially troubling sign.