The UN Security Council is to hold emergency talks after a suspected chemical attack in Syria left dozens of civilians dead and wounded.
The attack on a rebel-held town brought furious international reaction, with the US and other powers blaming the Syrian government for the deaths.
Officials in Damascus deny using any such weapons.
The attack will overshadow a conference in Brussels at which 70 donor nations will discuss aid efforts in Syria.
Delegates want to step up humanitarian access for thousands of civilians trapped by fighting.
Syria’s civil war has raged for more than six years with still no political solution in sight.
Nearly five million Syrians have fled the country and more than six million are internally displaced, the UN says. More than 250,000 people have been killed.
Wednesday’s emergency meeting of the UN Security Council was called by France and the UK as international outrage mounted over the suspected gas attack on Khan Sheikhoun in Idlib province on Tuesday.
Britain’s ambassador to the UN, Matthew Rycroft, described it as “very bad news for peace in Syria”.
“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,” he told reporters in New York.
Footage from the scene showed civilians, many of them children, choking and foaming at the mouth.
Witnesses said clinics treating the injured were then targeted by air strikes.
UK-based monitoring group the Syrian Observatory for Human Rights put the death toll at 58, including 11 children.
It was unable to say what chemical had been dropped but pro-opposition groups said it was believed to be the nerve agent Sarin.
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Aftermath of attack in pictures (Warning graphic images)
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