• The U.S. accused Russia of using chemical weapons against Ukrainian troops.
• The U.S. State Department also assessed that Russia has used "riot control agents," or tear gas, as a method of warfare in Ukraine, also in violation of the Chemical Weapons Convention.
• "The use of such chemicals is not an isolated incident," the State Department said.
• Russia responded to the accusations Thursday, calling them "unfounded" and unsubstantiated.

The U.S. has formally accused Russia of using chemical weapons against Ukrainian troops and announced late Wednesday that it is imposing more sanctions on Russian individuals and entities.

The U.S. State Department released a statement late Wednesday in which it accused Russia of violating the Chemical Weapons Convention, which bans the production and use of chemical weapons, by using the "choking agent" chloropicrin against Ukrainian forces.

"We make this determination in addition to our assessment that Russia has used riot control agents [tear gas] as a method of warfare in Ukraine, also in violation of the CWC. 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 statement said.

The Kremlin's press secretary, Dmitry Peskov, rebuffed the accusations when asked to comment by reporters Thursday.

"Yes, we saw the news on this matter. As always, such announcements sound absolutely unfounded and are not supported by anything. Russia has been and remains committed to its obligations under international law in this area," Peskov told reporters, news agency RIA Novosti reported.

Chloropicrin was used as a poison gas in World War I but is now more commonly utilized in agriculture as an insecticide. No longer authorized for military use, the choking agent inflicts injury mainly on the respiratory tract. It irritates the nose, throat and especially the lungs.

"When inhaled, these agents cause alveoli, air sacs in the lungs, to secrete fluid, essentially drowning those affected," the Organisation for the Prohibition of Chemical Weapons states.

In February, Ukraine accused Russia of using chemical weapons in more than 200 attacks on the battlefield in January alone. Russia denied the allegations and, in turn, has accused Ukraine of using chemical weapons. Neither has produced evidence for its claims, although soldiers have complained of injuries suspected to be a result of chemical agents.

The use of CS (or tear) gas, is also prohibited as a method of warfare; again, both have accused each other of using tear gas in the conflict.

The U.S. said Russia's use of such weapons showed an "ongoing disregard" for its obligations to the CWC, a global treaty in effect since 1997 that outlaws the production, stockpiling and use of chemical weapons.

Russia is among the 193 countries that ratified the convention, which calls on signatories to declare their stocks of banned chemical weapons and facilities, and to endeavor to destroy such stocks.

The continuing use of such weapons, the State Department said, "comes from the same playbook as its operations to poison Aleksey Navalny and Sergei and Yulia Skripal with Novichok nerve agents," referring to the alleged state-orchestrated nerve agent poisoning of Kremlin opponents in recent years. Russia denied any involvement in the incidents.

The U.S. said it is designating three Russian government entities "associated with Russia's chemical and biological weapons programs and four Russian companies providing support to such entities."

The Treasury Department, separately, is sanctioning three entities and two individuals "involved in procuring items for military institutes involved in Russia's chemical and biological weapons programs."