As it says Russia is ramping up the illegal use of riot control agents on the front to clear out trenches.

One infantryman says a drone dropped a tear gas grenade on his trench in the eastern front in January.

"I didn't get hit hard by [it] because I quickly put on my gas mask," he says.

"It's not lethal, but it disturbs and knocks you out. It makes it very difficult to carry out your duties once you've inhaled it."

More than two years since its full-scale invasion, Russian forces are advancing slowly but steadily in the east.

Kyiv has recorded around 900 uses of riot control agents by Russia in the past six months, a number that goes up every month, according to Colonel Serhii Pakhomov.

He's Ukraine's acting head of the military's atomic, biological and chemical defense forces.

Five hundred troops needed medical care after exposure - and at least one died after suffocating on tear gas, he says.

"In addition to demoralization, the person loses physical ability - he can't see, he can't breathe, everything is irritated. Yes, it is temporary, but it is the very moment the enemy can use to take over this position or another."

The international Chemical Weapons Convention bans the use of agents such as tear gas on the battlefield.

Both Russia and Ukraine are signatories.

LENNIE PHILLIPS: ".... that it's still being used, still being engaged, it is deliberate agreement by Russian seniority to break the chemical weapons convention."

Lennie Phillips is a senior research fellow on chemical weapons at the Royal United Services Institute in England.

He says the use of agents has been advertised on Russian TV.

And that there's little room for plausible deniability from troops in the field.

"I think it's fairly clear in my mind that the idea that local commanders are using it themselves without knowing that they're breaking the chemical weapons convention; I think that idea is now gone."

Russia's defense ministry did not respond to a request for comment.

Moscow has previously accused Ukrainian forces of using chemical weapons, something Kyiv denies.

Reuters has not been able to independently verify the use of banned chemical substances by either side.