Ukraine has accused Russia of unleashing “chemical agents” in an attack on the people of Mariupol, leaving soldiers and residents “dizzy and unable to breathe”.
Troops defending the port city of Mariupol are reportedly suffering respiratory failure, dizziness and other symptoms as a result of the “unknown substance”.
The unidentified agent is said to have been dropped on the southern city – which has seen some of the worst fighting since the war began – via a Russian drone, according to unverified reports from the city’s Azov regiment.
Addressing the reports, Foreign Secretary Liz Truss said it would be a “callous escalation” if Russia has used chemical weapons in its invasion of Ukraine.
She wrote on Twitter: “Reports that Russian forces may have used chemical agents in an attack on the people of Mariupol. We are working urgently with partners to verify details.
“Any use of such weapons would be a callous escalation in this conflict and we will hold Putin and his regime to account.”
It comes hours after the UK’s Ministry of Defence warned Vladimir Putin may resort to using devastating “phosphorous bombs” in his efforts to take hold of Mariupol and break the stalemate in Ukraine.
The MoD warned Russian forces had already used the devices in the Donetsk region of Ukraine.
“Russian forces prior use of phosphorous munitions in the Donetsk Oblast raises the possibility of their future employment in Mariupol as fighting for the city intensifies,” said the MoD on Sunday night.
White phosphorus is used for illumination at night or to create a smokescreen, but when it is deployed as a weapon it causes horrific burns.
Ukrainian President Volodymyr Zelenskyy has previously accused Russia of using the chemical.
The far-right Ukrainian Azov regiment said: “Russian occupation forces used a poisonous substance of unknown origin against Ukrainian military and civilians in the city of Mariupol, which was dropped from an enemy drone.
“The victims have respiratory failure… the effects of the unknown substance are being clarified.”
On Monday evening, Ivanna Klympush, a Ukrainian MP, tweeted: “Russia 1.5hr ago used unknown substance in #Mariupol.
“Victims experience respiratory failure, vestib.-atactic syndrome. Most likely chem.weapons!
“This is red line beyond which must destroy economy of despotism.We demand full embargo on all fuels from #RU &heavy weapons 2UA now!”
The alleged attack came just hours after a pro-Russian general in Donbas appeared to promote the idea of using chemical weapons.
They told state media Russia would “smoke the Ukrainian moles out of the underground”.
Meanwhile, the Ukrainian Parliament tweeted to say it has received reports of Russian forces firing “nitric acid” in the Donetsk region.
It warned local residents to wear “protective face masks soaked in soda solution”. It is not clear if the incidents are linked.
Mariupol’s mayor, Vadym Boychenko, said the city has suffered more than 10,000 deaths to date.
The port city has seen some of the most intense fighting since the invasion began at the end of February.
Mr. Boychenko also warned the toll could rise beyond 20,000 and claimed Russian forces are bringing mobile cremation equipment into the city to dispose of bodies.