Ukraine’s Azov regiment said that Russia had used a poisonous substance against Ukrainian troops in Mariupol, a besieged port on the Sea of Azov, writes kyivindependent.com.

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.