The Pentagon says it's monitoring 'deeply concerning' claims that Russia used chemical weapons in Ukraine

• Ukrainian politicians and troops accused Russia on Monday of using chemical weapons in Mariupol.
• The Pentagon said it was monitoring the reports closely but couldn't confirm them.

The Pentagon says it's closely monitoring several claims on Monday that Russia used chemical weapons on the besieged Ukrainian city of Mariupol.

"We are aware of social-media reports which claim Russian forces deployed a potential chemical munition in Mariupol, Ukraine," the Pentagon spokesman John Kirby said in a statement. "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," he said.

The use of chemical weapons by the Kremlin would be a major escalation of its invasion of Ukraine and could amp up pressure on the US or NATO to take direct action in the conflict.

Earlier Monday, the founder of Ukraine's far-right Azov Regiment, which has been fighting in Mariupol, said in a video on Telegram that Russia used chemical weapons on a steel plant where Ukrainian forces were dug in. The unit said an attack poisoned three people.

The same day, Lesia Vasylenko and Ivanna Klympush, both members of Ukraine's parliament, also spoke out on Twitter accusing Russia of using chemical weapons.

The accusations come as Eduard Basurin, a spokesman for the Russia-recognized separatist Donetsk People's Republic, suggested on Russian TV that Moscow should deploy "chemical forces" on Ukrainian troops, per The New York Times.

Ukrainian President Volodymyr Zelenskyy responded to Basurin's threat in a video address, calling his words a "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," Zelenskyy said. "We take this as seriously as possible."

British Foreign Secretary Liz Truss also tweeted Monday that the UK was working "urgently" to confirm details on the reports of chemical attacks.

"Any use of such weapons would be a callous escalation in this conflict and we will hold Putin and his regime to account," she wrote.

Mariupol, a port city in southern Ukraine cut off by Moscow-backed separatists and Russian troops, is now the site of one of the war's bloodiest conflicts.

Its mayor, Vadym Boychenko, said on Monday evening that more than 10,000 civilians had died in the siege so far, per the Associated Press, though that figure hasn't been independently verified. As of Monday, the United Nations confirmed 1,842 civilian deaths across Ukraine as a result of the war.