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 ... Nobody's talking about a military response."
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."