Russian President Vladimir Putin on Sunday insisted that Moscow had abided by "rules and norms" when providing weapons to Syria and called on other G8 countries which are contemplating arming rebels to do likewise.

"We are not breaching any rules and norms and we call on all our partners to act in the same fashion," he said in a press conference in London following talks with British Prime Minister David Cameron.

Putin warned that countries supplying arms to forces fighting against the regime of Syrian President Bashar al-Assad would have their reputation tarnished after footage emerged last month of one rebel apparently eating the heart of a dead soldier.

Human Rights Watch and the Syrian opposition National Coalition condemned the video as "horrific".

"It is barely worth it (supplying arms) to support people who not only kill their enemies but open up their bodies and eat their internal organs in front of the public and the cameras," Putin said.

"Do you want to supply these people with arms?

"In that case this hardly has anything to do with the humanitarian values which have for centuries been preached in Europe," he added. "At least in Russia we cannot imagine this."

Cameron earlier said that Assad's departure was essential "to end Syria's nightmare".

The British prime minister is seeking to forge an international consensus on handling the unrest as he hosts the leaders of the world's top industrialised nations in Lough Erne, Northern Ireland, from Monday.

Washington has upped the ante on Syria by vowing to send military aid to rebel forces battling to topple Assad after saying it had proof that his regime had crossed a "red line" by using chemical weapons on a small scale.

Britain has yet to decide whether it will attempt to funnel arms to moderate rebels.

Russia, which has given the Syrian regime military support and ignored months of pleas from the West to rein in Assad, was dismissive of the US claims.

However, prime minister Cameron insisted that Sunday's talks had convinced him there was scope for agreement when the world's top leaders meet next week.

"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 Cameron.

Putin stressed that the crisis could only be solved through "diplomatic means" and said that hopes for a peace conference in Geneva had not yet "been finally buried."

Charity Oxfam called on US President Barack Obama and Putin to "throw all their political weight" into making the conference a success.

"Failure to find a peaceful solution and sending more arms to either side of the conflict will only increase the bloodshed and suffering," said the international aid agency.