Theresa May has described the chemical weapons incident in Syria as "despicable" and said if the Assad regime was found to be responsible, its backers such as Russia should intervene.
Speaking during a visit to Nottinghamshire, the Prime Minister, who has just returned from a tour of the Middle East, said: "Obviously the Organisation for Prohibition of Chemical Weapons will investigate this soon and establish clearly all the facts around this.
"It is a despicable attack.
"If it is the case that it has been conducted by the Assad regime, it shows the barbarism of that regime and what I would say is all those who are backing that regime, including Russia, need to use their influence to stop Assad from bombarding and dealing with his people in such a horrific way."
She spoke out after the Trump administration joined the chorus of those blaming President Bashar Assad's Syrian government for the attack, but Moscow, a key ally of Assad, insisted the assault was caused by a Syrian air strike that hit a rebel stockpile of chemical arms.
At the United Nations, US ambassador Nikki Haley warned that Donald Trump's administration would take action if the Security Council did not in response to the attack.
President Trump, speaking at the White House, said: "When you kill innocent children, innocent babies - babies, little babies - with a chemical gas that is so lethal, people were shocked to hear what gas it was, that crosses many, many lines."
The president declined to say what the US would do in response, but said his "attitude towards Syria and Assad has changed very much".
Earlier on Wednesday, Downing Street played down the prospect of military action in retaliation for the use of chemical weapons against civilians, insisting "nobody is talking" about an armed response to the atrocity.
But Foreign Secretary Boris Johnson said he had seen "absolutely nothing to suggest" the attack was carried out by anyone but the Assad regime.
Meanwhile, the UK-based Syrian Observatory for Human Rights said the death toll from the attack had risen to at least 72, including 20 children.