London’s top diplomat on Monday said that Britain is trying to verify reports that Russia has used chemical weapons in an attack on the besieged Ukrainian city of Mariupol.

“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,” Foreign Secretary Liz Truss wrote on Twitter.

Also read: ‘Impose tougher rules’: Zelensky warns Russia could use chemical weapons

“Any use of such weapons would be a callous escalation in this conflict and we will hold Putin and his regime to account.”

Ukrainian lawmaker Ivanna Klympush said Russia had used an “unknown substance” in Mariupol and that people were suffering from respiratory failure. “Most likely chem.weapons!” she tweeted.

Also read: Austrian Chancellor to be the first EU leader to meet Putin since Ukraine invasion

On Monday, Ukraine’s Azov battalion in a Telegram message had claimed a Russian drone dropped a “poisonous substance” on Ukrainian troops and civilians in Mariupol.

The force also claimed that people were experiencing respiratory failure and neurological problems, according to news agency AFP.

Also read: Volodymyr Zelensky determined for Ukraine’s victory against Russia

The battalion’s founder, Andrei Biletsky, said that three people were suffering effects from an unknown toxic substance.

“Three people have clear signs of poisoning by warfare chemicals, but without catastrophic consequences,” he said in a video address on Telegram.

Also read: Ukraine’s Zelensky asks South Korea for military support in parliament address

Senior Donetsk separatist official Eduard Basurin had spoken of the possibility of using chemical weapons against the southern port city that has resisted Russian bombardment for weeks.

Basurin said the besieging forces could “turn to chemical troops who will find a way to smoke the moles out of their holes”, Russian news agency RIA Novosti quoted him as saying on Monday.