U.S. President Barack Obama held an hour-long video conference Friday evening with four of the G8 leaders who will be at the summit in Northern Ireland: British Prime Minister David Cameron, German Chancellor Angela Merkel, French President Francois Hollande and Italian Prime Minister Enrico Letta.
Prime Minister Stephen Harper, in Europe ahead of the G8 summit, did not participate in that discussion.
Ending the two-year bloodshed in Syria, which has killed at least 93,000 people, has assumed a new sense of urgency at the annual meeting of the leaders of the world's eight wealthiest countries now that the United States is getting more involved in the conflict.
It is expected Obama and other G8 leaders will press Russian President Vladimir Putin to relent on his country's persistent blocking of proposed UN sanctions against the Assad regime.
Russia's foreign minister, Sergey Lavrov, and others in Putin's inner circle have questioned claims by the United States that it has proof Assad's forces used chemical weapons — including the nerve agent sarin — against the rebels.
Lavrov told U.S. Secretary of State John Kerry in a telephone call Friday that the "accusations put forth by the United States to Damascus about the use of chemical weapons are not supported by trustworthy facts," according to a Russian Foreign Ministry statement.
Lavrov also cautioned that the deepening U.S. involvement would be "fraught with escalation in the region," the statement said.
Speaking on Friday in Paris, Harper said he accepts the U.S. claims.
"We share the view of our allies, I think, based on the evidence before us, that there have been uses of chemical weapons in Syria by the regime," Harper said.
"And as you know, the position of Canada on the regime is clear: we want to see Assad depart power and we want to see a regime that is representative of the entire population of Syria, which the Assad regime in its present form can never be."
Hollande, who spoke alongside Harper, said the rebels must be held accountable for how they use the arms and ammunition supplied by the U.S.
"We must exert some military pressure," Hollande said Friday.
"This is the reason why France, on a number of occasions, has set some principles while asking the Syrian opposition to be particularly clear as to its approach and the use of chemical weapons."
Harper, meanwhile, has other items on his to-do list ahead of the G8.
He arrived in Ireland on Saturday as he continues his European trip. While in Dublin, Harper will meet with Irish Prime Minister Enda Kenny and Deputy Prime Minister Eamon Gilmore.
Talks are expected to focus on trade — specifically the free-trade pact Canada is trying to broker with the European Union.
It's believed Ireland and France are pushing back against Canadian demands for more duty-free access to European markets. Harper has stopped in both countries on the pre-G8 leg of his trip.
He'll stay in Dublin until Monday, when he heads to Northern Ireland for the G8 summit.
— With files from The Associated Press