The UK is "working urgently" to verify reports that chemical weapons were used when attacking the city of Mariupol by Russian forces, according to the foreign secretary.

Liz Truss tweeted: “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.”

The city’s Azov regiment reported soldiers were left dizzy and unable to breathe after a “poisonous substance of unknown origin” was dropped on them from a Russian drone, according to the Daily Mail.

It came hours after Mariupol’s mayor said more than 10,000 civilians have died in the Russian siege of his city and the death toll could surpass 20,000.

Britain is increasingly worried that Russia could use white phosphorus munitions in the bombardment of the city.

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.

Western officials think Russia wants to bring about the fall of Mariupol to both free up troops for the fight in the Donbas but also to create a route north for the Kremlin’s forces as they look to form a pincer movement on Ukrainian defenders in the east.

Officials have said Mr Putin will double or even possibly triple the number of Russian troops in the Donbas as the Russian president resorts to a “diminished” invasion strategy.

The amassing of troops, however, will not necessarily give Moscow an advantage over Ukraine, with Kyiv’s forces having had success in pushing back insurgents in the east of the country, they said.

Meanwhile, late on Monday (April 11) Ukraine’s parliament said Russian forces had fired on nitric acid tanks in Donetsk, with residents of the eastern city urged to prepare “protective face masks soaked in soda solution”.