The U.S. authorities have accused Russian soldiers of violating the Chemical and Biological Weapons Control and Warfare Elimination Act of 1991 (CBW Act) by deploying chloropicrin, a chemical poison gas that causes respiratory difficulties, to battle Ukrainians in the yearslong war that began in February 2022.

According to a statement released on Wednesday, the U.S. State Department determined that Russian forces were using chloropicrin —used in World War I—to incapacitate Ukrainian soldiers in regions where they seemed to be losing.

Chloropicrin, the U.S. said, was a “choking agent” with characteristics similar to that of tear gas but far more toxic. When inhaled, it can suffocate the nose, throat and lungs.

Against the CBW Act 1991 ratified by Russia, chloropicrin and tear gas were considered too poisonous for use in a war. The Act banned the use of both chemical substances, but the U.S. says Russia has deployed them on several occasions to weaken the Ukrainian army.

“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 on Wednesday.

But Russia’s ambassador to the U.S. Anatoly Antonov condemned the accusations and called them “odious and unsubstantiated,” The New York Times cited him as saying in Telegram.

Still, Ukraine says over 1400 of its soldiers have been impacted by the chemical agent, which has made it nearly impossible to stand strong against Russia.

The U.S. government said it would impose more sanctions against Russia as part of an effort to condemn the war.