He chaired a humanitarian meeting at the G20 summit in St Petersburg this morning and urged fellow leaders to dig deep to meet the UN's appeal for £900m for Syria and £1.9bn for neighbouring countries this year.
"This is a moral imperative. This is the big refugee crisis of our time. As the (UN) Secretary General has made clear, seven million people are in dire need and chemical weapons attacks have made this even more acute," he told them.
"A Syrian becomes a refugee every 15 seconds while we sit here at this conference. That is 5,000 fleeing their homes and becoming homeless while we are at this G20 summit."
About third of Syria's pre-war 20.8 million population has fled over the border or have been forced from their homes during the popular uprising against President Bashar al Assad's regime which is now in its third year, UN refugee agency data showed.
"This is a humanitarian crisis of unprecedented proportions in recent history," UN chief Ban Ki-moon said.
The extra £52m brings Britain's total aid contribution to the Syria crisis to £400m.
David Cameron has ruled out any prospect of agreement at the summit, saying that Russian president Vladimir Putin remains "miles away" from the truth of Mr Assad's use of chemical weapons on his own people.
Meanwhile the US has ordered all non-essential embassy staff and other American citizens to leave Lebanon due to security concerns as the debate over military action continues.
"The Department of State drew down non-emergency personnel and family members from Embassy Beirut due to threats to US Mission facilities and personnel," a statement on the Beirut embassy's website said.
It also reducing its diplomatic presence in Adana in southern Turkey.
Earlier Syria's state news agency SANA said the country's head of parliament has urged the US Congress to vote against military action targeting its regime.
"We urge you not to take reckless measures as you have the power to steer the United States from the path of war to that of diplomacy," it quoted parliament chief Jihad al Lahham.
Relations among world leaders have become heated over possible US military action against President Assad's regime.
The US and Russia have been unable to agree on a way forward and President Barack Obama said he had "hit a wall" with Mr Putin, who has warned the use of force without UN approval would be "aggression" and a violation of international law.
Mr Cameron confirmed that deep divisions were voiced at last night's four-hour official dinner and said that - despite evidence of nerve gas sarin found by US and UK scientists - Mr Putin is still far from accepting the regime's responsibility for the August 21 attack which killed hundreds of civilians in a Damascus suburb.
"This G20 was never going to reach conclusions on Syria," said the Prime Minister. "The divisions are too great."
Following a 35-minute face-to-face meeting with Mr Putin in the early hours of Friday, Mr Cameron said: "He says to me that he would like to see further evidence of regime culpability and we will go on providing evidence of regime culpability, as will the Americans and others, but I think it will take a lot to change his mind."
White House official Ben Rhodes said Russia did not have "anything to add" to the political debate and criticised the country for "refusing to take action".
The Interfax news agency reported on Friday that Russia was sending a third landing ship towards Syria.
Citing a source at navy headquarters, it said the ship left the Black Sea port of Sevastopol for the Eastern Mediterranean with "special cargo".
Russia, a staunch ally of Syria, also reportedly dismissed Britain as a "small island no one pays any attention to" as relations boiled over at the summit.
The alleged comments, thought to include a reference to Soviet oligarchs "buying Chelsea" and attributed to President Putin's spokesman Dmitry Peskov, were later denied by Russian officials.
US Ambassador to the UN Samantha Power told a news conference in New York that Russia has held the UN Security Council "hostage" over Syria, and Mr Obama's administration did not expect that to end.
"Even in the wake of the flagrant shattering of the international norm against chemical weapons use, Russia continues to hold the council hostage and shirk its international responsibilities," she said.
"What we have learned, what the Syrian people have learned, is that the Security Council the world needs to deal with this crisis is not the Security Council we have."
The summit was tasked with fostering strong, sustainable growth through encouraging long-term investment among nations, but has been overshadowed by the Syria crisis.
On Thursday, the Syrian government wrote to the US Congress urging its members not to support the use of military force.
The letter, seen by Sky News, asked Congress to "communicate with us through civilised dialogue rather than the language of fire and blood".