The US State Department said on Wednesday it had determined that Russia used "the chemical weapon chloropicrin" against Ukrainian forces.

"We have seen the news about this. As always, such accusations sound completely baseless and unsubstantiated," Kremlin spokesman Dmitry Peskov told reporters when asked about the accusations.

The US said that Moscow had violated the Chemical Weapons Convention (CWC).

Chloropicrin is an oily substance known as a choking agent that was widely used during World War I as a form of tear gas.

The US Centers for Disease Control and Prevention label it a "lung damaging agent" that can cause severe irritation to skin, eyes and respiratory systems.

Its use is specifically prohibited by the Organisation for the Prohibition of Chemical Weapons, the implementing body for 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 State Department said on Wednesday.

Moscow has signed and ratified the CWC, which outlaws the production and use of chemical weapons.

"Russia has been and remains committed to its obligations under international law," Peskov said on Thursday.