The call came after one of Assad's top generals defected.
Representatives of about 100 countries met in Paris, but Russia and China — allies of the Assad regime — weren't there. Russia and China are both holders of UN veto power.
"All of us must tighten up our sanctions," Foreign Affairs Minister John Baird said. "Russia is not on the sidelines. By its actions, it is enabling this regime to soldier on."
U.S. Secretary of State Hillary Clinton was even harsher in her criticism.
Clinton said Russia and China "must pay a price" for blocking UN sanctions that might force Assad to step down.
"I don't think Russia and China believe they are paying any price at all — nothing at all — for standing up on behalf of the Assad regime," she said.
"The only way that will change is if every nation represented here directly and urgently makes it clear that Russia and China will pay a price. Because they are holding up progress, blockading it. That is no longer tolerable."
Baird, echoing comments from Clinton, said "the presence of large quantities of chemical weapons should send a fear up our spine and the potential there is truly horrifying. We need to strongly and forcefully draw a red line that the international community cannot stand by if Assad goes there."
He also said Canada will contribute an additional $1 million toward humanitarian assistance.
Baird also critcized the United Nations for considering Syria as a candidate for the UN Human Rights Council, calling it a "sick joke."
"The fact that they could even be a candidate speaks to the huge challenges that the United Nations faces. But any notion that they would sit on that council, could be elected to sit on that council, would do irreparable damage to the United Nations and would be cause for significant concern," he said.
The stepped-up international pressure came as Syrian Brig.-Gen. Manaf Tlass, a member of the elite Republican Guards and a son of a former defence minister, defected. Tlass's move was the highest profile departure in 16 months of brutal government crackdowns and civil strife in Syria.
French Foreign Minister Laurent Fabius initially said that Tlass was headed for France, where he has family members, but he later backed away from that statement, saying he was uncertain where the general was headed.
Clinton hailed the departure of Tlass, saying "those with the closest knowledge of Assad's actions and crimes are moving away. We think that's a very promising development. It also raises questions for those remaining in Damascus, who are still supporting this regime."Suggest a correction