With leaders from the G8 countries headed into a two-day meeting that begins Monday, Mr. Harper was alone in publicly articulating the division with Russia – and did so bluntly. “I don’t think we should fool ourselves. This is G7, plus one. That’s what this is, G7 plus one,” Mr. Harper told reporters Sunday after meeting with Irish Prime Minister Enda Kenny in Dublin. “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.”

The decision by the United States to arm Syrian rebels has put Syria at the forefront of discussions and set up clashes with Russian President Vladimir Putin, the only G8 leader to support the Syrian regime.

Mr. Putin had his own harsh words for the U.S. and other countries that support the rebels, saying they are backing cannibals. “I think 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,” Mr. Putin said at a news conference in London after meeting British Prime Minister David Cameron.

He apparently was referring to a video that has circulated this month and purports to show a rebel eating the heart of a dead Syrian soldier, an act fiercely condemned by leaders of the main anti-Assad forces. “Are these the people you want to support?” Mr. Putin said. “Is it them who you want to supply with weapons?”

Mr. Cameron struck a conciliatory tone after the meeting with Mr. Putin, saying Russia and the other G8 countries can still find common ground on Syria. “What I take from our conversation today is that we can overcome these differences if we recognize 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.”

None of this will help U.S. President Barack Obama who will meet with Mr. Putin on the sidelines of the G8 on Monday in an attempt to reach some kind of understanding over Syria and keep alive the possibility of a peace conference in Geneva. “We still continue to discuss with the Russians whether there is a way to bring together elements of the regime and the opposition to achieve a political settlement,” said Ben Rhodes, a deputy U.S. national security adviser. “There are no illusions that that’s going to be easy.”

Mr. Obama’s task is even more difficult because of the gains the Syrian army has been making recently in the civil war, thanks largely to Iranian-backed Hezbollah fighters. That has emboldened Syrian President Bashar al-Assad who has dismissed U.S. claims his army has used chemical weapons, saying the allegations were based on fabricated evidence.

The Russians, who have questioned the U.S. evidence as well, are also in a stronger position. The decision by the U.S. to arm the rebels could clear the way for the Russians to supply missiles to the Syrian army. Mr. Putin also did not rule out the possibility of a peace conference, saying on Saturday that he remained committed to a political solution to the crisis.

Mr. Obama also has to deal with divisions among his allies. Britain and France have already accused Mr. al-Assad of using chemical weapons and they pushed the European Union to lift its embargo on providing weapons to the rebels. However, neither government has yet to agree to supply arms.

Mr. Cameron is in a difficult situation politically because most of the members of his coalition government do not support sending weapons. The French government has said it wants to discuss the issue of arming the rebels at the G8 and it prefers to have any no-fly zone sanctioned by the United Nations Security Council, something that is seemingly impossible given Russia’s veto power.

Mr. Harper has also balked at supplying arms, saying he is concerned about extremist elements among the rebels. “We want to see the opposition in Syria become more representative, less sectarian,” he said Sunday. “We are not in Canada, at the present time, ... contemplating arming the opposition in Syria. I understand, fully understand, why our allies would do that particularly given the recent actions by Russia, Iran and others. But our aid at the present time, our aid for now, will continue to be humanitarian.”

Mr. Obama has reached out to German Chancellor Angela Merkel and Italian Prime Minister Enrico Letta to discuss Syria, but they have yet to commit to sending arms. Japanese Prime Minister Shinzo Abe is also expected to keep the focus on humanitarian aid To the more than 1. 5 million Syrian refugees and plans to announce more funding for that at the G8.

G8 will focus on other issues, such as clamping down on tax evasion and introducing more transparency into offshore bank accounts. Here too divisions abound.

Mr. Cameron is eager to push G8 countries to adopt a registry of beneficial owners of companies, something he and some rights groups say will help crack down on corruption and tax evasion. On Saturday, Britain and its territories and dependents, including Bermuda, Cayman Islands, Jersey and Guernsey, agreed to develop such a registry and exchange tax more tax information. It’s not yet clear what information will be covered or if the registry will be public.

However, some G8 members, including Canada, are cool to the idea of creating a registry. Mr. Harper said Sunday that he supports the ideas in principle but that Canada would have to get its provinces onside.