Britain's foreign secretary, Boris Johnson, on Wednesday said all evidence suggested the government of President Bashar Assad was behind the deadly chemical attack in Syria on Tuesday.
"All the evidence I have seen suggests this was the Assad regime ... using illegal weapons on their own people,"Johnson told Sky News.
He called the attack, which is believed to have injured hundreds and killed more than 50 people, including many children, a "barbaric act."
Assad's regime has denied any involvement in the attack.
Johnson also said those responsible should be held accountable and he could not picture a Syrian future with Assad in power.
"This is a barbaric regime that has made it impossible for us to imagine them continuing to be an authority over the people of Syria after this conflict is over," Johnson said on arrival at a meeting of European Union foreign ministers in Brussels.
British Prime Minister Theresa May echoed the foreign secretary's sentiments. May said she was "appalled" by reports of the attack, according to AFP. "We condemn the use of chemical weapons in all circumstances," May said, adding, "I'm very clear that there can be no future for Assad in a stable Syria ... We cannot allow this suffering to continue."
The attack took place Tuesday in the northwestern Syrian province of Idlib in the town of Khan Sheikhoun. The number of casualties is still unclear as different organisations have been reporting conflicting numbers.
The head of the health authority in Idlib province told Reuters more than 50 people had been killed and 300 wounded. The Union of Medical Care Organizations said the death toll was at least 100.
The British-based Syrian Observatory for Human Rights said that following new airstrikes on the town on Wednesday, the attack had killed at least 72 people, including 20 children.
The initial attack caused many of the victims to choke and some to foam at the mouth and convulse.
Earlier, Johnson, who is in Brussels for a Syrian aid conference, said in a statement that the incident bore the "hallmarks of an attack by the regime, which has repeatedly used chemical weapons."
Moscow on Wednesday said the Syrian airstrike had hit a "terrorist warehouse" containing "toxic substances.""According to the objective data of the Russian airspace control, Syrian aviation struck a large terrorist warehouse near Khan Sheikhun," the Russian defence ministry said in a statement.
If confirmed, this would be the deadliest chemical attack in Syria since sarin gas killed hundreds of civilians near Damascus in 2013. Syria's civil war has been raging for six years and has killed almost half a million people and displaced close to half of the country's population.