A diplomatic breakthrough on the Syrian civil war at the G8 summit in Northern Ireland appeared unlikely when the Russian president, Vladimir Putin, chastised the west for considering arming Syrian rebels, saying they ate human organs. He said Russia by contrast was arming the legitimate government of Syria.

Speaking after a difficult meeting with Putin in Downing Street, David Cameron claimed both men were in agreement on the need to end the human catastrophe of the civil war. But there was little to suggest the two men made progress on how to convene a fresh Syrian peace conference in Geneva, let alone who should attend, or its agenda.

In icy exchanges at a press conference, Putin said: "You will not deny that one does not really need to support the people who not only kill their enemies, but open up their bodies, eat their intestines in front of the public and cameras. Are these the people you want to support? Is it them who you want to supply with weapons? Then this probably has little relation to humanitarian values that have been preached in Europe for hundreds of years."

Putin's remarks will find an echo on the Conservative benches, where there is strong resistance to arming the Syrian opposition. Cameron has argued that it is possible to arm the pluralist democratic elements of the opposition, and he too wants to drive al-Qaida from Syria.

The talks Putin followed a decision by President Barack Obama's administration to arm rebels trying to overthrow President Bashar al-Assad after the US said it had obtained proof that the Syrian government had used chemical weapons. Some of the proof was provided by British scientists at Porton Down, and Cameron has now accused Assad of committing war crimes.

Cameron admitted the Russians and the UK held different points of view, but said the two countries would put aside their differences and focus on the "common ground" – organising peace talks between the parties in Geneva. Putin said he fully shared Cameron's view that the civil war could be ended "only by political and diplomatic means".

Cameron added: "We both see the humanitarian catastrophe. We both see the dangers of instability and extremism. We both want to see a peace process and a transition. The challenge for the G8 … is to put aside some of these differences."

The Russian leader, who arrived an hour late for the talks, said he wanted to help broker a peace deal for Syria, and he hoped the G8 summit in Northern Ireland could advance that process. Putin insisted his government was "not breaching any rules" in supplying weapons to Bashar al-Assad's "legitimate government" and called on partner G8 countries to respect the same rules.

"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," said Cameron.

In a sign of deteriorating relations among the G8, the Canadian prime minister, Steve Harper, claimed Putin was backing Assad's "thugs".

"I don't think we should fool ourselves," he said. "This is G7 plus one. We in the west have a very different perspective on this situation. Mr Putin and his government are supporting the thugs of the Assad regime for their own reasons that I do not think are justifiable, and Mr Putin knows my view on that."

Cameron's argument that it was possible to keep arms supplies out of the hands of extremist elements of the rebels was flatly contradicted by the mayor of London, Boris Johnson, who warned that there would be no way to prevent weapons ending up in the hands of "al-Qaida-affliated thugs".

Writing in the Daily Telegraph, Johnson said some elements of the rebels were fighting "not for freedom but for a terrifying Islamic state in which they would have the whip hand – and yet there is no dodging or fudging the matter: these are among the Syrian rebels who are hoping now to benefit from the flow of Western arms".

Cameron was also under pressure from his coalition partners the Liberal Democrats to avoid dragging Britain into a military conflict. Nick Clegg, the Lib Dem leader, implied that the case for intervention was less clear cut than in Libya or Iraq.

The shadow foreign secretary, Douglas Alexander, said MPs of all parties were feeling unease. "For months Labour has called on the government to answer basic questions about their approach, such as how the prime minister would ensure that weapons supplied did not fall into the wrong hands, and how this step would help to de-escalate the conflict rather than prolong it.

"The G8 is a key window of opportunity for David Cameron to exert pressure on President Putin and it is vital that he uses the coming hours to do so."

Tory MP Julian Lewis said it would be "suicidal" for Britain to hand arms to an opposition the government admits includes extremist elements.

He told the BBC's Radio 4: "The reason it would be suicidal would be that in taking over Syria they would also inherit Syria's arsenal of weapons, including in particular the nerve gas which is the centre of so much attention.

"In the past we have gone to war because we feared that weapons of mass destruction might fall into the hands of al-Qaida and it would be absolutely crazy to assist al-Qaida to get their hands on the very sorts of weapons we must keep away from them at all costs.

"I have little doubt the prime minister would struggle to get this achieved by parliament, because so many think it is not in Britain's national interest."