A major aid-pledging conference for Syria is taking place in Brussels as outrage over a suspected chemical weapons strike in the war-torn nation continues.
The UK is co-hosting the gathering which is being attended by Foreign Secretary Boris Johnson and International Development Secretary Priti Patel.
The United Nations Security Council will hold an emergency meeting at 3pm UK time on Wednesday to discuss reports from Idlib of chemical attacks, at the request of the UK and France.
Opposition activists claim dozens of people died in the attack in a town in the northern province, with the British-based Syrian Observatory for Human Rights putting the death toll at 58, including 11 children.
Matthew Rycroft, the UK's ambassador to the UN, told reporters in New York the attack was "very bad news for peace in Syria".
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."
Ahead of the aid summit, Mr Johnson said: "It is vital that the international community continues to support the Syrian people.
"I am determined that the conference which the UK is co-hosting in Brussels steps up international assistance to the most vulnerable people affected by the conflict.
"The UK is playing a leading role, with the Prime Minister's announcement of a £1 billion package the latest element of our vital support."
The international gathering comes after Prime Minister Theresa May called for an investigation into a suspected chemical weapons attack in the Syrian civil war.
Mrs May called on Moscow to ensure Bashar Assad's regime is brought to an end.
She said: "I'm appalled by the reports that there's been a chemical weapons attack on a town south of Idlib, allegedly by the Syrian regime.
"We condemn the use of chemical weapons in all circumstances.
"If proven, this will be further evidence of the barbarism of the Syrian regime."