Russia and Ukraine have accused each other at the global chemical weapons watchdog in The Hague of using banned toxins on the battlefield, the organisation said on Tuesday.
The Organisation for the Prohibition of Chemical Weapons (OPCW) said that the accusations were "insufficiently substantiated" but added: "The situation remains volatile and extremely concerning regarding the possible re-emergence of use of toxic chemicals as weapons."
Neither Russia nor Ukraine has formally asked the OPCW to investigate the alleged use of chemical weapons, it said.
Last week, the U.S. said Russia had violated the international chemical weapons ban overseen by the OPCW by deploying the choking agent chloropicrin against Ukrainian troops and using riot control agents "as a method of warfare" in Ukraine.
It followed Ukrainian assertions in April that Russia had increased its use of tear gas in the trenches.
Russia denied the allegations.
Ukrainian officials on Tuesday did not immediately respond to a request for comment.
The OPCW said it had been monitoring the situation since February 2022, when Moscow invaded Ukraine.
Under the Chemical Weapons Convention, any toxic chemical used with the purpose of causing harm or death is considered a chemical weapon.