The United States is not in position to confirm reports of the use of chemical weapons in Ukraine but was working to determine what actually happened, U.S. Secretary of State Antony Blinken said on Tuesday.
Ukraine said earlier it was checking unverified information that Russia may have used chemical weapons while besieging the city of Mariupol.
"We're in direct conversation with partners to try to determine what actually has happened," Blinken told reporters, adding that it had been a focus of concern even before Russia moved its troops into Ukraine.
A senior U.S. defense official on Tuesday said much the same thing about Mariupol, adding that the United States had no information to support the movement of chemical agents by Russia in or near Ukraine.
U.S. President Joe Biden last month said Russia's unsubstantiated accusations that Kyiv had biological and chemical weapons suggested Russian President Vladimir Putin may be laying the groundwork to use them.
Chemical weapons production, use and stockpiling is banned under the 1997 Chemical Weapons Convention. Although condemned by human rights groups, white phosphorous is not banned under the convention.
Russia's defence ministry did not immediately respond to a Reuters request for comment. Russian-backed separatist forces in the east denied using chemical weapons in Mariupol, the Interfax news agency reported.
"There is a theory that these could be phosphorous munitions," Ukraine's Deputy Defence Minister Hanna Malyar said on Tuesday.
Russia's invasion of Ukraine has killed thousands and displaced millions.
Moscow calls its actions a "special operation" to destroy Ukraine's military capabilities and capture what it views as dangerous nationalists, but Ukraine and the West say Russia launched an unprovoked war of aggression.
(Reporting by Idrees Ali, Humeyra Pamuk, Daphne Psaledakis and Ismail Shakil; Editing by Chizu Nomiyama and Howard Goller)
