WASHINGTON – The United States and the United Kingdom are investigating accusations that Russian soldiers attacked the Ukrainian port of Mariupol with chemical weapons.

Three soldiers from Ukraine's Azov unit were hurt by "a noxious chemical" in an attack on Monday, according to the regiment.  Foreign Secretary Liz Truss of the United Kingdom said officials were working to "urgently" probe what she called the war's "callous escalation."  The Pentagon described the weapons' possible use as "very disturbing."

Western countries have warned that using chemical weapons would be a dangerous escalation of the conflict, and have vowed to respond forcefully if Russia uses them.

Hanna Maliar, Ukraine's Deputy Defense Minister, said the government was looking into the allegations, and that the first indication suggested phosphorous munitions had been used. Pro-Russian separatists in Donetsk disputed responsibility for the strike on Tuesday.

Russian soldiers dropped "a deadly chemical of unknown origin" during a drone attack on Mariupol's major Azovstal metals plant, said the Azov battalion, which has been prominently involved in fighting in Mariupol and has significant ties to the far-right.

It claimed that some of its fighters had sustained minor injuries, such as loss of breath.

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