Britain and the United States have accused Russia of using the toxic agent chloropicrin against Ukrainian troops in violation of the Chemical Weapons Convention (CWC).

The British foreign ministry said it had slapped sanctions on Kirillov and the Russian army's Radiological, Chemical and Biological Defence unit "for helping deploy these barbaric weapons".

It also sanctioned two Russian defence ministry laboratories "for providing support for the development and deployment" of chemical agents on the frontline.

"The UK will not sit idly by whilst Putin and his mafia state ride roughshod over international law, including the Chemical Weapons Convention," said Foreign Secretary David Lammy.

"Russia's cruel and inhumane tactics on the battlefield are abhorrent and I will use the full arsenal of powers at my disposal to combat Russia's malign activity."

Chloropicrin is an oily liquid with a pungent odour known as a choking agent that was widely used during World War I as a form of tear gas.

The US Centers for Disease Control and Prevention labels it a "lung damaging agent" that can cause severe irritation to skin, eyes and respiratory systems.

The Organisation for the Prohibition of Chemical Weapons (OPCW) specifically prohibits its use.

Russia has said it no longer possesses a military chemical arsenal but the country faces pressure for more transparency over the alleged use of toxic weapons.

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

In June, Ukraine accused Russia of increasing frontline attacks using prohibited hazardous chemicals and had registered more than 700 cases of their use the previous month.

Moscow has signed and ratified the CWC, which outlaws the production and use of chemical weapons.

The sanctions subject Kirillov to an asset freeze and ban him from entering the United Kingdom. They also freeze the assets of the sanctioned entities.

The UK has been one of Ukraine's biggest backers in its war against Russia's full-scale invasion of its neighbour, committing to delivering pound3.0 billion ($3.9 billion) of military aid every year for as long as needed.