The UK's under-secretary for the armed forces, James Heappey, has revealed that "all possible options are on the table in terms of how the west might respond" if it is confirmed Russia used chemical weapons on Ukraine.
"Any use of such weapons would be a callous escalation in this conflict and we will hold Putin and his regime to account." -Liz Truss
Heappey did not confirm what kind of reaction it would prompt, but said it would "certainly be effective."
"There's real value in some ambiguity," the minister said.
Talking to BBC Radio 4's Today show, he said a reaction could take place outside of NATO involvement.
UK Foreign Secretary Liz Truss also vowed to "hold Putin and his regime to account" if reports that Russian troops used chemical weapons in their latest attack on Mariupol are verified.
"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," wrote Truss on Twitter late on Monday.
"Any use of such weapons would be a callous escalation in this conflict and we will hold Putin and his regime to account."
The UK government has confirmed it is working with allies to verify the reports, also cited by Ukraine's President Volodymyr Zelensky.
On Monday, Ukrainian authorities voiced reports that Russian troops had dropped a drone containing toxic chemicals over the southern port city of Mariupol.
Leader of Ukraine's Azov volunteer regiment Andriy Biletsky said that three people in Mariupol had suffered "poisoning by warfare chemicals, but without catastrophic consequences."
The claims have not yet been verified and officials in Mariupol are waiting for official information from Ukraine's military on the reports.
The city's mayor also reported that over 10,000 civilians have died so far in Mariupol since the Russians invaded Ukraine.
On Tuesday, Zelensky mentioned reports saying Russian troops might be using chemical weapons to break the defense of Mariupol.
"Today the occupiers issued a new statement, which testifies to their preparation for a new stage of terror against Ukraine and our defenders," Zelensky said.
"One of the mouthpieces of the occupiers stated that they could use chemical weapons against the defenders of Mariupol. We take this as seriously as possible."
Mariupol, a strategic city on the Black Sea considered to be one the main targets of the Russian invasion, has been the stage of terrible devastation since the war broke out and is now possibly the most bombed and damaged city in Ukraine.
The city's defenders are still resisting Russian attacks. "Mariupol is the heart of this war today," Zelensky said in an interview last week.
"It's beating. We're fighting. We're strong. And if it stops beating we will be in a weaker position."
Addressing South Korea's National Assembly in a video speech on Monday, Zelensky said the Russians have "completely destroyed Mariupol and burned it to ashes."
Moscow has not yet commented on the claims. Back in March, Russia was escalating claims, proven false, that the U.S. and Ukraine were developing chemical or biological weapons to use against Russian troops.
U.S. and Ukrainian officials feared at the time that the claims were an attempt from Russia to justify the use of chemical and bioweapons in Ukraine by Russian forces.
Newsweek has reached out to Russia's Ministry of Defense for comment.