The Foreign Secretary made the comments following reports that Russian forces may have used chemical agents in the port city of Mariupol. Fighters with Ukraine’s Azov regiment, who are defending the region against Putin’s forces, claimed Russian troops had “used a poisonous substance of unknown origin against Ukrainian forces and civilians”.
No evidence was given to back up the claim and civic chiefs stressed it had not been confirmed and they were waiting for more information.
Ms Truss said any confirmed use of such weapons would be a “callous escalation” of the war.
She added that Putin and his troops would be held accountable if chemical weapons were utilised.
She tweeted: “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.
“Any use of such weapons would be a callous escalation in this conflict and we will hold Putin and his regime to account.”
Britain’s armed forces minister James Heappey also spoke about the topic on Sky News and warned there would be consequences for Russia if confirmed.
He said that “all options are on the table” if Russia had deployed chemical weapons during its onslaught on Ukraine.
READ MORE: Putin prepares tens of thousands of troops for brutal new onslaught
He went on to say the West would use a “well-considered” response but stopped short of saying exactly how the US, UK and their allies might react.
He added: “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.”
Mr Heappey and Ms Truss’s comments come after Ukraine’s president Volodymyr Zelensky used his nightly video address to citizens to warn that Russia could deploy a chemical attack.
However, he did not say Russian troops had already done so.
He said: “We treat this with the utmost seriousness.
“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.”
US defence department spokesman John Kirby also said the reports were unconfirmed but that Washington “will continue to monitor the situation closely.”