Boris Johnson has dismissed Syrian government denials that it was responsible for a chemical weapons strike which killed dozens of people, including children.
The UK-based Syrian Observatory for Human Rights said the death toll had risen to at least 72, including 20 children.
Witnesses have claimed the attack was carried out by jets operated by the Russian and Syrian governments.
But the Syrian government “categorically rejected” this, instead blaming rebels and accusing them of making it up to frame the regime.
And Russia said the town of Khan Sheikhoun was exposed to chemicals from a rebel arsenal hit by a Syrian air strike.
Arriving for a major aid-pledging conference for Syria in Brussels, Mr Johnson told reporters: “I’ve seen absolutely nothing to suggest, or rather to lead us to think, that it’s anything but the regime.
“All the evidence I have – and there may be more to come out of this – all the evidence I’ve seen suggests that this was the Assad regime who did it in the full knowledge that they were using illegal weapons in a barbaric attack on their own people.”
Mr Johnson added: “You cannot go on with a regime that’s willing to use illegal weapons against its own people, a regime that’s killed hundreds of thousands of its own people.
“What’s needed now is a political process to get rid of that regime and give the people of Syria a chance.”
Mr Johnson said he would like to see “those culpable pay a price”, adding: ” I think what it confirms to everybody is that this is a barbaric regime that has made it impossible for us to imagine them continuing to be in authority over the people of Syria after this conflict is over.”
He added: “We need to wait and see exactly what has happened. If this is confirmed to have been another chemical weapons attack by the Assad regime, with or without the complicity of the Russians…
“I think what it shows is that this is a government that has absolutely no compassion for its own people that has put itself beyond the pale.”
His Secretary of State Rex Tillerson said it was “clear that this is how Bashar al-Assad operates: with brutal, unabashed barbarism”.
He added: “Those who defend and support him, including Russia and Iran, should have no illusions about Assad or his intentions. Anyone who uses chemical weapons to attack his own people shows a fundamental disregard for human decency and must be held accountable.”
He said: “This is clearly a war crime and I call on the Security Council members who have previously used their vetoes to defend the indefensible to change their course.
“We need to wait for the full investigation to take place, but, as I said, this bears all the hallmarks of a regime attack.
“It is only the regime that have the necessary equipment to deliver an attack like this and I’ve seen the same reports that you have about what the weapon might have been.”