Downing Street has played down the prospect of military action in retaliation for the use of chemical weapons against civilians in Syria, insisting "nobody is talking" about an armed response to the atrocity.
Britain and France are bringing forward a resolution at the United Nations Security Council condemning the attack in the largely opposition-held Idlib province, which is believed to have killed at least 72 people, including 20 children.
Foreign Secretary Boris Johnson said he had seen "absolutely nothing to suggest" the attack was carried out by anyone but the government of Syrian president Bashar Assad.
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.
Russia said the town of Khan Sheikhoun was exposed to chemicals from a rebel arsenal hit by a Syrian air strike.
Ahead of Wednesday's emergency meeting of the Security Council, a Number 10 source told reporters travelling with Theresa May in Saudi Arabia: " We want everybody to condemn what happened yesterday and let's see what happens.
"We hope that everybody will condemn what has happened and that there will be agreement that those responsible should be brought to justice."
The source said that "nobody's talking" about a military response, adding: "There's a UN resolution this afternoon. The Prime Minister ... made a very strong statement condemning this attack, and Britain has brought forward a UN Security Council resolution that will be debated this afternoon."
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."
US President Donald Trump also blamed the Assad regime, saying the attack was "reprehensible and cannot be ignored by the civilised world".
The UK-based Syrian Observatory for Human Rights said the death toll had risen to at least 72, including 20 children.
Speaking at the aid conference, Mr Johnson said the Syrian regime was engaged in tactics of "starve or surrender".
He said: "As we sit here in Brussels it is still the case that the regime is preventing the UN from delivering aid to millions of Syrians - besieging over 475,000 people with the aim of starving them into submission.
"Together we should make clear our abhorrence of the regime's tactic of starve or surrender. We must remind all sides of their obligations contained in numerous UN resolutions to allow aid to reach all who need it wherever they may be.
"The people of Syria are today paying a price for our collective inaction over the last five years and decisions we took.
"We cannot now undo those mistakes but we can and we must work together to alleviate their suffering. To help Syria's neighbours and prepare Syria for the moment when peace finally returns."
Liberal Democrat leader Tim Farron said: "The Government must immediately consider new sanctions on Russia and how the international community can create a humanitarian zone in Syria.
"Trump may be happy to blame it on Obama and look the other way, but our Prime Minister must rise above this and defend international law.
"We will not win against Isil in Syria without ending the civil war, and we will not end the civil war if we sit idly by as civilians are gassed.
"Britain and our allies must not stand by whilst this horror unfolds, we know from history there is a price of doing nothing and trying to look the other way.
"It is not one we can countenance paying."
Speaking at a press conference, Mr Johnson said no one could reasonably object to the UN resolution.
"We in the UK, together with our French friends, have called an emergency meeting of the UN Security Council. We have tabled a resolution which I hope absolutely everybody feels able to sign up to.
"Because all we are saying is that there should be condemnation of that chemical weapons attack, and secondly, that there should be a thorough and urgent international investigation, and I don't think anybody could possibly, reasonably, oppose such a resolution."
Labour leader Jeremy Corbyn has called for a peaceful resolution and said those responsible should answer for their crimes in the criminal courts.
Speaking at a press conference on Wednesday he said: "What has happened is appalling, illegal and wrong and I want Syria to join the chemical weapons convention.
"The way forward has to be a political dialogue, it has to be a peace process, it has to be a return to the Geneva dialogues, that's what I'm calling on people to do.
"I think we have to put the pressure on Russia, we have to put the pressure on others who are supplying weapons into the theatre of war, we have to bring about a political solution.
"The use of chemical weapons is unacceptable in this world and I want Syria to be held accountable."
He added: "I think that there should be a report made to the UN and the international criminal court for the use of chemical weapons.
"It's disgusting, disgraceful, illegal and wrong what has happened.
"There has to be a peace process, there has to be all the political parties to have a resumption of a meaningful ceasefire and in the longer-term a political solution in Syria so that refugees can return home and live in safety and peace."