The West will have “all options on the table” if Vladimir Putin’s forces are found to have used chemical weapons in an assault on the Ukrainian port of Mariupol, a UK defence minister said.
Western officials are examining claims that a chemical agent was used by Russian forces in a drone attack on the besieged city.
Armed forces minister James Heappey said the use of chemical weapons was “beyond the pale” and if the reports were accurate there would be a response from the UK and its allies.
“It’s important to recognise that there are all sorts of ways in which these things could be used, from the use of tear gas which is effectively a riot-control measure, all the way through to utterly devastating lethal chemical weapons systems, so I don’t think it’s helpful to be too binary about the situation because these are highly nuanced,” he told Sky News.
But he added “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”.
Foreign Secretary Liz Truss said the UK was working urgently with partners to verify details of the attack.
The Azov regiment, a unit with far-right links which is defending Mariupol, claimed the substance was delivered by a drone.
The Kyiv Independent news website reported that Azov leader Andriy Biletsky said that three people have clear signs of chemical poisoning.
Ms 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.”
Shadow foreign secretary David Lammy said: “If chemical weapons have been used as part of Vladimir Putin’s already barbaric and illegal invasion of Ukraine, this would represent an appalling escalation and amount to a war crime.
“These deeply disturbing reports must now be verified as a matter of urgency.
“If true, Putin and his criminal cronies must be held to account.”
In his nightly address, Ukraine’s President Volodymyr Zelensky said the threat of chemical weapons being used was taken “as seriously as possible”.
Russian forces have concentrated their efforts on Mariupol in the south and the Donbass region of eastern Ukraine, after apparently abandoning efforts to seize capital Kyiv.
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.
A UK defence intelligence summary published on Tuesday suggested fighting will intensify in eastern Ukraine over the next two to three weeks.
“Russian attacks remain focused on Ukrainian positions near Donetsk and Luhansk with further fighting around Kherson and Mykolaiv and a renewed push towards Kramatorsk,” the Ministry of Defence said.
“Russian forces continue to withdraw from Belarus in order to redeploy in support of operations in eastern Ukraine.”
Western officials think Russia wants to bring about the fall of Mariupol to free up troops for the fight in the Donbas and 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.
The Russian leader has been forced to “diminish considerably” the plan in Ukraine, one official said.
There are suggestions Mr Putin wants to take the Donbas region before May 9 – when Russia traditionally marks the Soviet Union’s Second World War victory against Nazi Germany with military parades in Moscow – in an attempt to claim victory for his so-called “special operation”.
Meanwhile, late on Monday Ukraine’s parliament said Russian forces had fired on nitric acid tanks in Donetsk, with residents of the eastern city being urged to prepare “protective face masks soaked in soda solution”.