Ukrainian President Volodymyr Zelensky sharply denounced the claims. (File)
A senior US official on Sunday dismissed Russian claims that Ukraine is preparing to use a dirty bomb as "transparently false," while the Pentagon chief "rejected any pretext for Russian escalation."
Russian Defense Minister Sergei Shoigu made the allegation about a dirty bomb in a round of telephone conversations with Western defense chiefs.
Ukrainian President Volodymyr Zelensky sharply denounced it, calling the allegation a Russian pretext for just such an attack. "The world should react as harshly as possible," he said.
A statement Sunday from US National Security Council spokeswoman, Adrienne Watson said, "We reject reports of Minister Shoigu's transparently false allegations that Ukraine is preparing to use a dirty bomb on its own territory.
"The world would see through any attempt to use this allegation as a pretext for escalation."
Shoigu conducted a round of telephone consultations with counterparts from Britain, France and Turkey, all NATO members, after first speaking Friday with Austin.
In those calls, Shoigu conveyed "concerns about possible provocations by Ukraine with the use of a 'dirty bomb,'" the Russian Defense Ministry said.
Shoigu and Austin spoke Sunday, in what the Pentagon said was a followup call requested by Russia to their call Friday.
"Secretary Austin rejected any pretext for Russian escalation and reaffirmed the value of continued communication amid Russia's unlawful and unjustified war against Ukraine," said a statement afterward from Pentagon press secretary Pat Ryder.
When Shoigu and Austin spoke on Friday, it was only their second call since Moscow sent troops to Ukraine on February 24.
Zelensky on Sunday sharply dismissed the claim that Kyiv was preparing to use a dirty bomb.
"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 on social media.
"I believe that now the world should react as harshly as possible."
"Even the very Russian threat of nuclear weapons -- and even more so against our country, which has given up its nuclear arsenal... is a reason for both sanctions and for even greater strengthening of support for Ukraine," said Zelensky.
A so-called dirty bomb is designed to contaminate a wide area with radioactive material, making it dangerous for civilians. It does not involve a nuclear explosion.
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