On Ukraine’s front lines, the country’s soldiers report what the statistics show: a continued use of chemical weapons, mainly tear gas, whose deployment on the battlefield is illegal.

The US and UK take the accusations further and say Russia is using another toxic agent, chloropicrin, which was first used in the trenches of the first world war.

Behind the effort are troops from the Russian Defense Ministry’s Radioactive, Chemical and Biological Defense Unit, known as RKhBZ, the Guardian reports.

The US and UK say it is a specialist unit, partly responsible for the use of chemical weapons on the battlefield, and was headed by Igor Kirillov, who was killed in Moscow by a bomb hidden on a skate in an attack carried out by Kiev.

Ukraine says there have been 4,800 documented uses of chemical weapons by Russia since the full-scale invasion began in February 2022, and more than 2,000 people have been hospitalized. Several media reports, citing soldiers’ accounts, say small drones are being used to drop gas grenades on Ukrainian positions, suffocating soldiers or forcing them to abandon their positions.

In May, the US said Russia was using tear gas and other agents on the battlefield, including chloropicrin.

Banned by the Organization for the Prohibition of Chemical Weapons (OPCW) on the battlefield, chloropicrin is a pale yellow liquid that, when released near humans, causes choking, dizziness, severe eye pain, skin irritation, vomiting and, in the most severe cases, death.

The US State Department said: “The use of such chemicals is not an isolated incident and is likely due to the desire of Russian forces to dislodge Ukrainian forces from fortified positions in order to make tactical gains on the battlefield.” He then announced sanctions against the Russian unit RKhBZ.

In October, the United Kingdom took the issue even further. David Lammy, the foreign secretary, and John Haley, the defense secretary, accused Russia of violating the chemical weapons treaty and imposed sanctions on both the unit and Kirillov himself. “Russia’s cruel and inhumane tactics on the battlefield are appalling,” Lammy said, saying chloropicrin was among the chemical agents used.

A Kyiv Independent study published in August, based on data released by Ukrainian officials, showed that the use of chemical agents on the battlefield increased in the first half of this year. It said there were 81 cases in December 2023, rising to 715 in May, only to decrease again as winter approached, reaching 166 cases, according to Ukrainian data.

Hours before Kirillov’s death, Ukraine’s internal security service SBU published its own report online that concluded that the Russians are using chemical weapons “mainly in the most critical combat areas.

Russia has denied using chemical weapons in Ukraine, with Kirillov himself denying the reports. “Russia destroyed all stockpiles of chemical weapons in September 2017,” the general said in February, although allegations of novichok use on Sergei Skripal and Alexei Navalny suggest otherwise. Kirillov also claimed that Russia found a chemical weapons laboratory near Avdiyka in Ukraine, although this claim has never been verified.

Until recently, official investigations by the OPCW from both sides have been limited, in part because in order for the international body to act it must be invited by either Russia or Ukraine to investigate. The use of chloropicrin has not been proven by the OPCW, but last month, after a visit to Ukraine, the watchdog said there was evidence of the use of a type of tear gas.

“Analyses by OPCW laboratories, conducted separately and independently, confirm that the samples collected from the trench contain the agent 2-chlorobenzylidenemalonitrile, known as CS,” the report said.