Stephen Harper is urging Russia to stop thwarting international efforts to slap sanctions on Bashar al-Assad's "murderous" regime in Syria.

The Prime Minister targeted Moscow with public remarks Wednesday, calling on Russia to climb on board international efforts to impose binding sanctions on the Assad regime.

Russia, a stalwart ally of Damascus, has used its permanent seat on the UN Security Council to block measures such as global economic sanctions on Syria. Canada is working as part of a co-ordinated diplomatic offensive, led by the United States, that's aimed at pressuring Moscow into relenting.

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"We encourage Russia and others to join with us to apply binding sanctions against what is a murderous regime," Mr. Harper told the Commons. "This is unacceptable to Canadians and I believe the broader international community."

Mr. Harper didn't name additional countries but his use of the word "others" appears to be a reference to China which also has a permanent seat on the Security Council and has stood with Syria.

Canada's efforts come as Mr. al-Assad's bloody crackdown on a 15-month-old uprising morphs into full-blown civil war.

U.S. Secretary of State Hillary Clinton this week accused Russia of supplying Syria with helicopter gunships that could be used against civilians. On Wednesday, however, Russia's foreign minister Sergei Lavrov denied the charge.

The Financial Times quoted him as saying Moscow was merely "completing contracts that were signed and paid for a long time ago. All of them are contracts for what are solely air-defence systems."

Mr. Harper's comments Wednesday followed a question from NDP Leader Tom Mulcair.

"We have all witnessed the horrors in Syria, where innocent children are now being targeted by the Assad regime. Canadians feel the pain of the Syrian people," the NDP Leader said.