World leaders are facing calls to put in place a new multi-billion pound deal to support the millions of Syrian refugees who have fled the fighting in their country's brutal civil war.
Aid agencies are gathering in London ahead of a major international funding conference on Thursday hosted by Britain, Germany, Norway and the United Nations.
The UN is appealing for 7.7 billion dollars (£5.4 billion) to fund aid operations for the millions of people displaced in Syria and the neighbouring countries over the coming year.
It comes after last year's UN appeal for 2.9 billion dollars (£2 billion) was 60% under-funded.
US president Barack Obama spoke on Tuesday by phone to David Cameron, and told the Prime Minister that the US will soon announce "significant new contributions" to assist Syria's humanitarian crisis.
The White House said US Secretary of State John Kerry would announce the aid at the summit.
Both leaders are calling for better access for humanitarian aid to reach Syria.
A coalition of more than 90 aid agencies and humanitarian organisations - including Oxfam and Amnesty International - is pressing for governments to go further with a new, multi-year funding plan.
They also calling for measures to enable neighbouring countries - such as Turkey, Jordan and Lebanon - to allow refugees to work and to ensure they have access to essential services such as healthcare and children's education.
Former foreign secretary David Miliband, who now heads the International Rescue Committee said: "Syrians are facing a war without law and a war without end.
"The latest harrowing scenes from the besieged town of Madaya and the rising pressure on neighbouring states need to spur political leaders to act.
"The relentless suffering of the Syrian people should be a global call to action for humanitarian assistance to alleviate suffering and for political action to bring the war to an end."
The meeting comes as King Abdullah of Jordan - which has seen an influx of hundreds of thousands of refugees- warned his country could no longer cope with the pressures of accepting so many people.
"Sooner or later, I think, the dam is going to burst," he warned in a BBC interview. "The psyche of the Jordanian people, I think it's gotten to boiling point."