Ukrainian authorities are checking unverified information that Russia may have used chemical weapons while besieging the southern port city of Mariupol, Deputy Defense Minister Hanna Malyar said Tuesday.

"There is a theory that these could be phosphorous munitions," Malyar said in televised comments, adding, "Official information will come later."

Russia's Defense Ministry did not immediately respond to a request for comment.

Russia-backed separatist forces trying to wrest complete control of Mariupol denied using chemical weapons in comments carried by Russian news agency Interfax.

Mariupol's city council wrote on the Telegram messaging service that it was not yet possible to examine the area where the poisonous substance had allegedly been used because of enemy fire. It added that the city's civilian population had minimal contact with the unspecified poison but that Ukrainian soldiers had come into closer contact with it and were now being observed for possible symptoms.

Ukrainian President Volodymyr Zelenskyy warned on Monday that Russia could use chemical weapons in Ukraine.

Britain and the United States have said they are aware of reports that Russia may have already used chemical agents in Mariupol. Britain said it was working with partners to verify the reports.

Russia has previously accused Ukraine of preparing to use chemical weapons, without providing evidence.

Last month the Kremlin said U.S. talk of Russia using such weapons was a tactic to divert attention away from awkward questions for Washington.

Russia's invasion of Ukraine has killed thousands and displaced millions. Moscow calls its actions in Ukraine 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.