The UN's chemical-weapons watchdog says it is "concerned" about unconfirmed reports of the use of chemical weapons in the Ukrainian port of Mariupol.
The Organization for the Prohibition of Chemical Weapons (OPCW) is closely monitoring the situation, it said.
"The Technical Secretariat of the OPCW...is concerned by the recent unconfirmed report of chemical weapons use in Mariupol, which has been carried in the media over the past 24 hours," the OPCW said in a statement.
Ukrainian President Volodymyr Zelenskiy said it would not be possible to draw firm conclusions about whether Russian forces have used chemical weapons in Mariupol.
Members of the U.S. Congress who have also been monitoring the situation said during a trip to Poland on April 12 said that the United States and its allies would not stand by if chemical weapons were used in Ukraine.
The United States is investigating reports that a poisonous substance has been dropped in Mariupol, the lawmakers said, but they cautioned that determining the nature of any attack in Mariupol could take time.
"We're taking those reports seriously and I know the United States government and others are trying to determine if that did indeed occur," Representative Jason Crow (Democrat-Colorado) said.
Crow said the administration "has been very clear that the use of chemical weapons will not be tolerated."
U.S. Secretary of State Antony Blinken also addressed the reports about chemical weapons coming from Mariupol.
"We're not in a position to confirm anything, I don't think Ukrainians are either," Blinken told reporters.
He added that Washington had "credible information" that Russian forces may use a variety of riot-control agents, including tear gas mixed with chemical agents. These would be aimed at causing symptoms to weaken or incapacitate Ukrainian fighters and civilians in Mariupol, he said.
"We share that information with...Ukraine and other partners," Blinken said. "And we're in direct conversation with partners to try to determine what actually is happening."