Russian President Vladimir Putin has said both sides of the Syrian war have "blood on their hands" as he issued a blunt warning to the West not to arm the rebels.
Following talks in Downing Street with Prime Minister David Cameron, Mr Putin said the rebels "eat the organs" of their enemies.
He was referring to footage that emerged last month of one rebel purportedly eating the heart of a dead soldier.
Human Rights Watch and the Syrian opposition National Coalition condemned the video as "horrific".
Mr Putin said: "One does not really need to support the people who not only kill their enemies, but open up their bodies, eat their organs in front of the public and cameras."
"Are these the people you want to support? Are they the ones you want to supply with weapons?
"Then this probably has little relation to the humanitarian values preached in Europe for hundreds of years," he said at a news conference with Mr Cameron.
The US decided last week to send weapons to the rebels, saying Syrian President Bashar al Assad's forces had crossed a "red line" by using chemical weapons.
The talks with Mr Cameron came before the G8 summit, which starts tomorrow in Lough Erne in Northern Ireland.
Mr Putin, however, stressed that he wanted to achieve a diplomatic solution to the crisis.
He said that he believed the G8 summit was "one of the most appropriate means" to seek an end to the conflict.
Mr Cameron earlier said that Assad's departure was essential "to end Syria's nightmare".
The Prime Minister acknowledged differences between the West and Moscow, but insisted that the talks with Mr Putin had convinced him there was scope for agreement at the G8.
"What I take from our conversation today is that we can overcome these differences if we recognise that we share some fundamental aims: to end the conflict, to stop Syria breaking apart, to let the Syrian people decide who governs them and to take the fight to the extremists and defeat them," he said.
"We will use the opportunity of having G8 leaders together to try and build on this common ground.
"We must work together to do everything we can to bring this dreadful conflict to an end," added Mr Cameron.
Britain has yet to decide whether it will attempt to funnel arms to moderate rebels.
Mr Putin stressed that the crisis could only be solved through "diplomatic means" and said that hopes for a peace conference, tentatively scheduled for next month in Geneva, had not yet "been finally buried".
Finding a political path out of Syria's war is expected to dominate the G8 talks.
The meeting gathers the leaders of Canada, France, Germany, Italy, Japan, Russia, the US and the UK.