David Cameron has hailed international pledges totalling more than £7 billion to ease the plight of Syrian refugees, amid warnings the latest onslaught by President Bashar Assad's forces could trigger a new mass exodus.
At the end of an international pledging conference in London, the Prime Minister said the money promised by more than 60 countries would give hope to the millions of people displaced in the five-year civil war.
But there was a stark warning by Turkish Prime Minister Ahmet Davutoglu of a fresh "humanitarian disaster" after regime forces - supported by Russian air strikes - cut off the northern city of Aleppo.
He said that a supply corridor between the city and the Turkish border had been overrun, and accused Assad of preparing to mount a "siege of starvation" against Aleppo's 300,000 inhabitants.
Ten thousand people had already fled to the border at Kilis in the hope of finding a "safe haven", he said, while another 30,000 had "escaped" from refugee camps in the area and were heading in the same direction.
The offensive against Aleppo has already halted the latest attempt to re-start international peace talks in Geneva after just two days.
Mr Davutoglu bitterly denounced the involvement of Russian warplanes, saying that in the past three days they had mounted 351 air strikes against the rebels - including civilian targets - and none against Islamic State - also referred to as Daesh.
"We have to be shoulder-to-shoulder against those committing war crimes," he said.
"In the last three days Russian warplanes bombarded Aleppo, and regime forces on the ground with foreign fighters - usually that means Daesh, but foreign fighters on the side of the regime - they attacked Aleppo.
"The logistic corridor from Turkey to Aleppo has been cut off. This corridor is now under the invasion of these foreign fighters and regime forces by the support of Russian warplanes.
"What they want to do today in Aleppo is exactly the same as what they did in Madaya before - the siege of starvation."
Mr Cameron urged Russia to use its influence with the Assad regime to end the "indiscriminate" attacks, particularly the use of barrel bombs against civilian targets.
"We must urgently redouble our efforts to prevent the intolerable levels of violence against civilians, ensuring all parties in the conflict bring an immediate end to the ongoing violations of international humanitarian law," he said.
"Russia should support steps towards a ceasefire as envisaged by the Vienna process and mandated by the UN Security Council."