The US State Department said Russia had used riot control agents "as a method of warfare" in Ukraine, and that the use of such chemicals was "not an isolated incident" but rather a tactic driven by Russian forces' desire to dislodge Ukrainian forces from fortified positions.

Chloropicrin is listed as a banned choking agent by the 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.

In a related development, the US Treasury Department has announced sweeping sanctions aimed at crippling Russia's military and industrial capabilities. The sanctions target nearly 300 entities in Russia, China, and other countries accused of supporting President Vladimir Putin's invasion.

The US sanctions aim to punish companies that help Moscow acquire weapons for its war in Ukraine, and also target Russian government entities and companies involved in the country's chemical and biological weapons programs.

Russia has claimed it no longer possesses a military chemical arsenal, but faces pressure for greater transparency over the alleged use of toxic weapons.

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Meanwhile, Ukrainian President Volodymyr Zelenskyy has urged the US to speed up weapons deliveries, delayed for months due to political wrangling in Congress. The US has pledged to speed up deliveries after lawmakers approved $61 billion in new aid for Ukraine.

"We urgently need a significant boost in military capabilities to empower our soldiers," President Zelenskyy said. "It's not Russian air power and assault operations that should dominate the front line, but our Ukrainian initiative - our air defence, our artillery, our drones."

(With inputs from agencies)