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

Both Russia and Ukraine have accused each other of using chemical weapons in the nearly three-year-long conflict, with Kyiv's Western allies likewise claiming Moscow has employed banned weapons.

"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."

CS gas is non-lethal but causes sensory irritation including to the lungs, skin and eyes.

- Upcoming meet -

The discovery of the riot gas in Ukraine is now set to be a major talking point next week, when states that have signed up to the convention gather in The Hague for an annual meet to discuss compliance.

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

Ukraine said the samples had been collected following an incident on September 20 near the village of 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".

- 'Grave concern' -

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 treaty.

The OPCW said in May that information it had received until then on the alleged use of chloropicrin in Ukraine was "insufficiently substantiated". The Kremlin called the claims "baseless and unsubstantiated".

A month later Kyiv accused Russia of stepping up frontline attacks using "hazardous chemical compounds", mainly CS gas, by Russian forces, registering 715 cases in May.

In October, Britain sanctioned the chemical and biological arm of Russia's armed forces and its commander Igor Kirillov over the alleged use of chemical weapons in the war.

OPCW director-general Fernando Arias "expressed grave concern" over the findings.

"All 193 OPCW Member States, including the Russian Federation and Ukraine, have committed never to develop, produce, acquire, stockpile, transfer or use chemical weapons," he said in a statement.

"States Parties to the CWC have declared that any use of chemical weapons is totally unacceptable and would violate the legal norms and standards of the international community," Arias said.