Ukraine's Azov regiment said on April 11 that Russia had used a poisonous substance against Ukrainian troops in Mariupol, a besieged port on the Sea of Azov.
Azov leader Andriy Biletsky said that three people have clear signs of chemical poisoning. He added that there are no "disastrous consequences" for their health.
The claim followed a call by Russia's proxies in the Donbas to use chemical weapons against Azov.
If confirmed, this is the first known use of chemical weapons by Russia during its aggression against Ukraine. Western leaders have pledged to step up their response to Russian aggression in case of a chemical attack.
Azov said that the poisonous substance had been distributed by a drone. Its victims have shortness of breath and vestibullocerebellar ataxia, the regiment said.
Earlier on April 11, Eduard Basurin, a spokesman for Russia's proxies in Donetsk, made a statement on Russia's attempts to capture Mariupol's well-fortified Azovstal steel mill, which is held by Azov. Mariupol has been besieged by Russia since late February.
"There are underground floors (at Azovstal), and that's why it makes no sense to storm this object now," he said. "We could have a lot of our soldiers killed, and the enemy won't suffer casualties. That's why currently we should figure out how to block this mill and find all ways in and out. And after that we should ask our chemical forces to find a way to smoke these moles out of their holes."
Russia has falsely accused Ukraine of having chemical and biological weapons programs. Western authorities have said this might indicate Russia's intentions to use weapons of mass destruction and put the blame on Ukraine.
Both U.S. President Joe Biden and NATO Secretary General Jens Stoltenberg have said that the use of chemical weapons by Russia would be a red line and fundamentally change the nature of the conflict.
Russia and its allies have a track record of using chemical weapons.
Russia's Federal Security Service used novichok, a chemical weapon, in an attempt to poison opposition leader Alexei Navalny in 2020, as well as several other opposition figures, according to investigative journalists.
Syrian dictator Bashard al-Assad, a major ally of Russia, has used chemical weapons multiple times during Syria's civil war since 2012, with Russia defending and whitewashing his actions.
