James Heappey made the comments to Sky’s Kay Burley – as foreign secretary Liz Truss said the UK was “working urgently” to check the reports that Russia used chemical agents in the besieged city of Mariupol.
Ms Truss said any confirmed use of such weapons in Mariupol would be a “callous escalation” of the war.
Mr Heappey, there had been no verification of the reports but warned that Britain and its allies would respond to any deployment by Russia of chemical weapons.
“Let’s be clear, if they are used at all then President Putin should know that all possible options are on the table in terms of how the West might respond,” he said.
Volodymyr Zelenskyy, Ukraine’s president, used his nightly video address to warn that Russia could deploy outlawed chemical weapons, but he did not say they had already done so.
“We treat this with the utmost seriousness,” he said.
“I would like to remind world leaders that the possible use of chemical weapons by the Russian military has already been discussed.
“And already at that time it meant that it was necessary to react to the Russian aggression much harsher and faster.”
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Key developments:
• Ukraine’s President Volodymyr Zelenskyy has pleaded for more weapons from the West, particularly for the defence of Mariupol
• Austrian Chancellor Karl Nehammer met Russian President Vladimir Putin and warned that an offensive in Ukraine’s east was “being prepared on a massive scale”
• Allegations of rape, executions, and other war crimes are still coming to light in areas around Kyiv, where the Russians pulled out in recent weeks. Russia says these are Ukrainian and Western provocations
• Reuters data shows that, while many Western countries have tightened sanctions on Russia, India – lured by massive discounts – has purchased more Russian crude oil since the beginning of the invasion than it did for the whole of last year
• Russia’s foreign minister Sergei Lavrov said Russia would not stop fighting for any new round of peace talks. Talks were last held on 1 April
• The UK Ministry of Defence said Russian shelling has continued in Donbas but Ukrainian forces have beaten back several assaults and destroyed Russian military vehicles and equipment
