An Organisation for the Prohibition of Chemical Weapons (OPCW) convention bans the use of CS gas and other toxic weapons in war zones.

"The results of the analyses of these samples conducted by two OPCW designated laboratories ... indicate that both a grenade and a soil sample... contained the riot control agent known as CS," the OPCW said in a statement.

It is the first time the use of a riot control gas has been confirmed in areas where active fighting is taking place in Ukraine, the OPCW said.

Based in The Hague, the OPCW's Chemical Weapons Convention strictly bans the use of riot control agents including CS, a type of tear gas, outside riot control situations when it is used as "a method of warfare."

Ukraine asked the watchdog to send a technical team and handed over three samples to them on a visit last month

Ukraine said the samples had been collected following an incident on September 20 near the village Illinka in Ukraine's central Dnipropetrovsk region.

"The team collected related documentation and digital files as well as testimonies from first-hand witnesses and also received three samples collected from Ukraine: one grenade shell and two soil samples from a trench."

The evidence handed over by Ukraine to the OPCW during the visit enabled it to "corroborate... the chain of custody of the three samples collected from a trench in Ukraine located along the confrontation lines with the opposing troops, had been maintained," the OPCW said.

It stressed however that the report did "not seek to identify the source or origin of the toxic chemical."

Britain and the United States have accused Russia of using the toxic agent chloropicrin as well as riot control agents since its 2022 invasion of Ukraine in violation of the CWC .

Russia and Ukraine have also accused each other of using chemical weapons.