A destroyed armored vehicle during ongoing conflicts in the city of Mariupol, Ukraine, under the control of the Russian military and pro-Russian separatists on April 9. Photo: Leon Klein/Anadolu Agency via Getty Images

The Pentagon is closely monitoring social media reports claiming Russian forces deployed chemical weapons in Mariupol, Ukraine, Defense Department press secretary John Kirby said on Monday.

Why it matters: Moscow has a long history of using chemical weapons, and the White House has warned that Russian forces may be preparing to “use chemical or biological weapons in Ukraine.”

Driving the news: The original report was made on Telegram, where the far-right nationalist group Azov Regiment posted a message alleging that Russian forces used “a poisonous substance of unknown origin,” according to CNBC.

What they’re saying: “We are aware of social media reports which claim Russian forces deployed a potential chemical munition in Mariupol, Ukraine,” Kirby said Monday evening in a statement to outlets including Axios. “We cannot confirm at this time and will continue to monitor the situation closely.”

  • “These reports, if true, are deeply concerning and reflective of concerns that we have had about Russia’s potential to use a variety of riot control agents, including tear gas mixed with chemical agents, in Ukraine.”

Of note: Eduard Basurin, a spokesperson for the Kremlin-backed separatist Donetsk People’s Republic said later on Russian TV that Russia’s military should use “chemical forces” in Mariupol and “smoke out the moles,” per the New York Times.

  • Ukrainian President Volodymyr Zelensky responded to Basurin’s comments in a televised address Monday night, saying: “Today, the occupiers issued a new statement, which testifies to their preparation for a new stage of terror against Ukraine and our defenders.
  • “One of the mouthpieces of the occupiers stated that they could use chemical weapons against the defenders of Mariupol. We take this as seriously as possible,” he added.
  • Zelensky had yet to comment on the Azov Regiment’s claims.

The big picture: British Foreign Secretary Liz Truss also said the U.K. government was working “urgently” to verify the alleged use of chemical agents.

  • “Any use of such weapons would be a callous escalation in this conflict and we will hold Putin and his regime to account,” she tweeted.
  • France has already sent crime scene and forensic experts to help investigate possible war crimes in parts of Ukraine previously occupied by Russian forces.

By the numbers: Russia’s unprovoked invasion has forced more than 4.3 million people to flee Ukraine.

Go deeper: Why allegations of chemical weapons use are hard to investigate

Editor’s note: This article has been updated with comment from Basurin and Zelensky and with details on the number of Ukrainian children displaced.