All options would be on table in response to any use of chemical weapons in Ukraine by Russia, James Heappey, the Armed Forces minister, has said.

“There are some things that are beyond the pale, and the use of chemical weapons will get a response and all options are on the table for what that response could be,” Mr Heappey told Sky News.

His comments came as foreign minister Liz Truss said Britain was working with its partners to verify the details of reports Russian forces may have used chemical agents in an attack on Mariupol.

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

Last night the Azov Brigade claimed chemical weapons had been used in the southern city. But an aide to the city’s mayor says that has not been confirmed and that they are “waiting for official information from the military”.

Russian-backed separatist forces did not use chemical weapons in their attempts to take full control of the city of Mariupol despite Ukrainian allegations to the contrary, Eduard Basurin, a separatist commander, told the Interfax news agency on Tuesday.

Ukraine’s Deputy Defence Minister Hanna Malyar said earlier on Tuesday that Kyiv was checking unverified information that Russia may have used chemical weapons while besieging the southern Ukrainian port city.

Ukraine’s Deputy Prime Minister Iryna Vereshchuk said nine humanitarian corridors had been agreed for Tuesday to evacuate civilians, including from the besieged city of Mariupol by private cars.

Ms Vereshchuk said in a statement that five of the nine evacuation corridors were from Ukraine’s Luhansk region in the east of the country, which Ukrainian officials have said is under heavy shelling.