David Cameron has called on world leaders to step up aid for the Syrian refugees amid fears that a fresh onslaught by President Bashar Assad's forces will drive a new exodus from the country.
Opening international talks on the refugee crisis in London, the Prime Minister called for a new approach focusing on helping people displaced by the fighting to rebuild their lives in the region.
The meeting came as parallel peace talks in Geneva stalled after just two days following the launch of a new offensive by Assad's forces – backed by Russian air strikes – against rebels in northern Syria.
Turkish Prime Minister Ahmet Davutoglu told the conference that "civilians, schools and hospitals" had been hit during the "air bombardment" and the attacks on the city of Aleppo.
He said that there are already 10,000 refugees on the Turkish border trying to escape the fighting, while up to 70,000 people at the refugee camps and 300,000 in Aleppo could follow.
United Nations Secretary General Ban Ki-moon expressed concern at the way his special envoy Staffan de Mistura had been forced to "pause" the Geneva talks so soon after they had started.
"It is deeply disturbing that the initial steps of the talks have been undermined by the continuous lack of sufficient humanitarian access, and by a sudden increase of aerial bombing and military activities within Syrian," Mr Ban said.
Following talks with US Secretary of State John Kerry and other foreign ministers ahead of the main conference, Foreign Secretary Philip Hammond said it was essential that talks resumed as soon as possible.
"We recognise that it's difficult for the regime to be at the table talking to the opposition, it's difficult for the opposition to talk to the regime when their people at home are being killed through bombing and other forms of attack," he said.
"But we have to continue with this process because it is the only way to get a solution to the disaster which is engulfing Syria."
Mr Kerry said that he had had a "robust" discussion with his Russian counterpart Sergey Lavrov and called on Moscow to abide by a UN Security Council resolution requiring immediate access for humanitarian agencies and an end to attacks on civilians.
"It could not be more clear," said Mr Kerry. "That is an obligation that is not tied to talks, it is an obligation accepted by all parties in the United Nations resolution. Russia voted for that, Russia has responsibility - as do all parties - to live up to it."
The UN, which is co-hosting the talks with the UK, Germany, Norway, and Kuwait, is calling for the international community to commit 7.7 billion US dollars (£5.4 billion) to fund its humanitarian relief work for the refugees for the coming year.
With an estimated 13.5 million people in need of assistance, Mr Ban said that the level of contributions so far had not kept pace with the scale of the crisis.
"Even if, by some miracle, the conflict ends tomorrow, the enormous humanitarian and development needs will continue for years and even decades," he said.
Mr Cameron, who committed an additional £1.2 billion of UK aid over the next four years, urged the 70 nations represented at the talks to make the "greatest possible pledge".
"We are facing a critical shortfall in life-saving aid that is fatally holding back our humanitarian efforts," the Prime Minister said.
"After years of conflict we are witnessing a desperate movement of humanity as hundreds of thousands of Syrians fear they have no alternative than to put their lives in the hands of evil people-smugglers in search of a future."
He called for assistance for Syria's neighbours - Turkey, Lebanon and Jordan - to enable them to open up job opportunities for the estimated 4.6 million refugees in their countries, helping them remain in the region.
He also appealed for a commitment to ensure that all the refugee children in those countries had a school place by the end of 2016.