The Prime Minister insisted today that Britain and Russia could "overcome" their "very big differences" and come up with a course of action for ending the conflict in Syria.
His comments came after a meeting with Russian President Vladimir Putin in Downing Street.
During a joint press conference in which those differences were made all too clear, David Cameron described Syrian President Bashar al-Assad as a "murderous dictator" and contended that he "has to go".
Putin countered that with a blunt warning to Britain not to try to arm Syrian rebels who "eat the organs" of their enemies.
Putin also strongly defended the supply of arms by Moscow to the "legitimate" government of President Assad, insisting Russia was acting fully in accordance with international law.
His uncompromising stance marked a difficult start to the the G8 summit, which opens tomorrow in Lough Erne in Northern Ireland, where Mr Cameron is hoping to make progress on a range of international issues.
In a radio interview tonight, the Prime Minister acknowledged that finding agreement on Syria would be a "very tough ask".
Meanwhile he is facing growing opposition at home to any move to join US president Barack Obama in arming the rebels - with MPs from all sides lining up to condemn the idea.
Mr Cameron had invited Mr Putin to Downing Street ahead of the G8 specifically so they could discuss the Syrian issue which has emerged as the primary bone of contention between them.
The meeting opened with a moment of reconciliation, with the two leaders handing medals to the veterans of the Arctic convoys who helped keep Russia supplied during the dark days of the Second World War.
At a joint news conference afterwards, Mr Cameron acknowledged there were "very big differences" between their analyses on the conflict, but insisted they could be overcome.
In contrast, Mr Putin was unsparing when he was asked about previous comments by Mr Cameron that those who armed the regime had "the blood of the children of Syria" on their hands.
"The blood is on the hands of both parties ... One should hardly back those who kill their enemies and eat their organs," he said, referring to a video purporting to show a Syrian rebel commander biting into the heart of a soldier.