Russia's state Investigative Committee said on Monday it was examining the alleged use of chemical weapons by Ukrainian forces near the towns of Soledar and Bakhmut.
Ukraine's Defence Ministry did not immediately reply to requests for comment on the allegation, which was not accompanied by any publicly released evidence.
The Investigative Committee said the Donetsk People's Republic - one of Russia's proxies in the territories it has seized and occupied in eastern Ukraine - had reported the use of chemical weapons by Ukrainian drones near the two locations.
"As a result, servicemen of the Russian Armed Forces are experiencing a deterioration in their health and characteristic symptoms of poisoning," it said, without providing details or naming the alleged substance.
Since the start of its invasion nearly a year ago, Russia has repeatedly warned that Ukraine might be preparing to use non-conventional weapons, including biological weapons or a radioactive dirty bomb. No such attack has materialised.
Ukraine and its Western allies rejected those accusations but saw them as a possible prelude to a "false flag" attack, meaning Russia might itself resort to such tactics but seek to blame Ukraine. Russia has dismissed that claim.
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