INTERNATIONAL: The US and UK said they are working urgently to confirm unverified reports that Russia used chemical weapons in the southern city of Mariupol.

Ukrainian MP Ivanna Klympush tweeted that an "unknown substance" had been used in the city, which causes respiratory failure and movement disorders.

"We cannot confirm at this time and will continue to monitor the situation closely," Pentagon spokesperson John Kirby said in response. "These reports, if true, are deeply concerning and reflective of concerns that we have had about Russia's potential to use a variety of riot control agents, including tear gas mixed with chemical agents, in Ukraine."

"We are working urgently with partners to verify details,' UK Foreign Secretary Liz Truss said of the reports. "Any use of such weapons would be a callous escalation in this conflict and we will hold Putin and his regime to account."

Ukrainian President Volodymyr Zelenskyy said during a Monday evening address that the Russian military could use chemical weapons, but did not say that they have already done so during the invasion.