Foreign secretary Liz Truss and Armed Forces minister James Heappey said the UK was working with allies to verify reports that Russian forces may have used chemical agents in an attack on Mariupol.
Mr Heappey said any use of chemical weapons “will get a response” – and suggested some countries may choose to act outside of Nato. “There are some things that are beyond the pale,” he said.
The defence minister told Sky News said that if chemical weapons are used “then president Putin should know that all possible options are on the table in terms of how the west might respond”.
Asked what the options would include, Mr Heappey said “ambiguity” was important – but added that any response “would certainly be effective, but it would also be well considered”.
Challenged on BBC Radio 4’s Today programme if a response could involve direct Nato involvement, Mr Heappey said: “All options are on the table … It’s important to have some ambiguity.”
He added: “It’s not for any government minister from any individual country to pledge Nato involvement. It’s perfectly possible that a response to chemical weapons use could happen outwith Nato.”
Mariupol’s Azov regiment reported that 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.
Mr Truss said “any use of such weapons would be a callous escalation in this conflict and we will hold Putin and his regime to account”, but made clear the UK was still trying to verify reports.
“The Ukrainian system, as you’ve seen from president Zelensky, are only referring to the fact that there are reports, they themselves haven’t yet been able to confirm to us that they have been used,” Mr Heappey told Sky News.
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 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”.
The Foreign Office said it was “shocked” by reports of mass graves being found in the village of Buzova outside Kyiv. Local officials said bodies showing “evidence of execution” had been discovered following the Russian withdrawal.
Buzova is near Bucha, another town where atrocities were discovered – although Russia has claimed the scenes from the aftermath of its occupation were staged.
The UK will work with allies to “investigate war crimes and ensure justice is done”, a Foreign Office spokesman said.