The UK is "concerned" that Vladimir Putin may use chemical weapons in Ukraine after his troops bombed a children's hospital in the besieged city of Mariupol.
Liz Truss warned it would be a "grave mistake" on Putin's part if he were to do so, adding to the litany of errors he has made since invading Ukraine.
The Foreign Secretary's chilling remarks comes hours after Western officials expressed "serious concerns" that Russia could use chemical weapons.
In a frank but brief interview with CNN, Ms Truss said: "We are very concerned about the use of chemical weapons. Now of course we've seen Russia use these weapons before.
"That would be a grave mistake on the part of Russia adding to the grave mistakes that have already been made by Putin."
Yesterday an official, who spoke on condition of anonymity, said: “We’re seeing some of the disinformation come out of Moscow about non-conventional weapons.
“We saw this in Syria, the Russians starting that talk when they or their proxies were about to use those weapons there.”
Worries are mounting among allied governments after Moscow was seen “setting the scene” for such an attack.
Russia-backed Syria was repeatedly accused of using chemical weapons in the country's bloody war, but Russia vetoed a 2017 UN resolution that would have looked into who was responsible.
The Organisation for the Prohibition of Chemical Weapons found people were exposed to sarin gas in 2017 in the Khan Shaykhun area but did not identify who was responsible.
Boris Johnson branded Russia "depraved" after Ukraine accused Putin's forces of bombing a children's and maternity hospital in the besieged port of Mariupol - during a supposed ceasefire to let civilians escape.
The city council warned "the destruction is colossal", while President Volodymr Zelensky called it an "atrocity".
Mr Johnson tweeted: "There are few things more depraved than targeting the vulnerable and defenceless.
Ms Truss today also condemned the attack in Mariupol adding: "We believe war crimes are being committed. we've referred the case to the international criminal court we never expected war like this to be taking place in Europe again.
"This is why we need such a tough international response.
"Putin has to lose in Ukraine, that is vitally important. It's appalling the suffering we're seeing in Ukraine."
Western officials say Russia’s military advance remains “very slow” with “minimal progress” over the last 24 hours - partly due to the Ukrainians’ resistance and partly due to logistical and execution problems with Russian troops.
Russia's artillery strike on a Mariupol hospital which left three people dead including a child was a war crime, the Government has declared.
Pregnant women were seen being carried out of the shelled facility in the southern Ukrainian coastal city.
Pressed on whether he thinks the attack constitutes a war crime, Defence Minister James Heappey: "Yes - if you deliberately target a piece of civilian infrastructure like a hospital, yes.
“If you use indiscriminate artillery into an urban area without due regard for the reality, you could hit a protected site like a hospital, then that too in my view is."
Amid growing fears Moscow could order chemical weapon attacks, he warned the Kremlin that their past use has "triggered an international response".
"I don't think it's helpful to get into any firm commitment right now about where that red line sits, but I think President Putin needs to be very clear that when other countries have used chemical weapons it has caused an international response,” he said.
"President Putin needs to be clear that the use of chemical weapons is just the most despicable thing that anybody can imagine.”
