The United States on Wednesday accused Russia of violating the international chemical weapons ban by deploying the choking agent chloropicrin against Ukrainian troops and using riot control agents “as a method of warfare” in Ukraine.

“The use of such chemicals is not an isolated incident and is probably driven by Russian forces’ desire to dislodge Ukrainian forces from fortified positions and achieve tactical gains on the battlefield,” the State Department said in a statement.

The Russian Embassy in Washington did not immediately respond to a request for comment.

Chloropicrin is listed as a banned choking agent by the Hague-based Organization for the Prohibition of Chemical Weapons (OPCW), which was created to implement and monitor compliance with the 1993 Chemical Weapons Convention (CWC).

German forces fired the gas against Allied troops during World War I in one of the first uses of a chemical weapon.

Earlier this month, Reuters reported the Ukrainian military as saying Russia has stepped up its illegal of use riot control agents as it presses its biggest advances in eastern Ukraine in more than two years.

In addition to chloropicrin, Russian forces have used grenades loaded with CS and CN gases, the Ukrainian military says. It says at least 500 Ukrainian soldiers have been treated for exposure to toxic substances and one was killed by suffocating on tear gas.

While civilians usually can escape riot control gases during protests, soldiers stuck in trenches without gas masks must either flee under enemy fire or risk suffocating.

The State Department said it was delivering to Congress its determination that Russia’s use of chloropicrin against Ukrainian troops violated the CWC.

Moscow’s use of the gas “comes from the same playbook as its operations to poison” the late opposition leader Alexey Navalny in 2020 and Sergei Skripal and his daughter Yulia in 2018 with the Novichok nerve agent, the statement said.

Russia denied involvement in both cases.

The department also determined that Russia has breached the CWC’s prohibition on the use of riot control agents as a method of warfare, the statement said.

It said it was sanctioning three Russian state entities linked to Moscow’s chemical and biological weapons programs, including a specialized military unit that facilitated the use of chloropicrin against Ukrainian troops.

Four Russian companies that support the three entities were also sanctioned, it said.

The sanctions freeze any U.S. assets belonging to the targeted entities and generally prohibit Americans from doing business with them.

Separately, the U.S. Treasury imposed sanctions on three entities and two individuals involved in purchasing items for Russian military institutes involved in the country’s chemical and biological weapons programs.

The sanctions were among new measures announced by the United States on Wednesday targeting Russia over its 2022 invasion of Ukraine.

The CWC bans the production and use of chemical weapons. It also requires the 193 countries that have ratified the convention, which include Russia and the U.S., to destroy any stocks of banned chemicals.

The State Department was expected to convey its determination that Russia has violated the CWC to the OPCW.

Russia and Ukraine have accused each other of breaching the treaty in OPCW meetings. But the organization says it has not been formally asked to open an investigation into the use of prohibited substances in Ukraine.

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