Foreign Affairs Minister John Baird, U.S. Secretary of State Hillary Clinton and senior officials from about 100 other countries gathered in Paris to bolster support for a Syrian transition plan unveiled last week by UN mediator Kofi Annan.
The plan would force Assad into a ceasefire and then ease him from power.
Neither Moscow nor Beijing attended the Paris meeting. Baird later singled out Russia over China as the biggest obstacle standing in the way of Assad's ouster. The two countries are permanent members of the U.N. Security Council and in the past have used their veto powers to block sanctions and resolutions on Syria.
"I think we all acknowledge that Russia is the significant obstacle here," Baird told a teleconference.
"It is not merely blocking UN Security Council sanctions, it is enabling this regime to soldier on. They need to reflect on the role they want to play in the civilized world."
His words echoed Clinton's tough language toward U.N. Security Council members Russia and China. Baird told reporters that during the Paris meeting, Clinton was "as strong and as tough as I have ever seen her."
Clinton urged other countries to step up diplomatic pressure on Russia and China.
"What can every nation and group represented here do?" Clinton asked.
"I ask you to reach out to Russia and China, and to not only urge but demand that they get off the sidelines and begin to support the legitimate aspirations of the Syrian people."
Beijing and Moscow do not appear to think there are consequences for their actions, she added.
"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," Clinton 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 also had harsh words for the United Nations for considering Syria as a candidate for the UN Human Rights Council.
"It is outrageous and it is a sick joke," he said.
"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."
The watchdog group UN Watch cited a draft resolution presented this week in Geneva in which the United States and the European Union oppose Syria's candidacy for a seat on the 47-nation UN body in 2014.
Hillel Neuer, the executive director of UN Watch, said Syria may end up on the same Human Rights Council that has condemned the violence at the hands of the Assad regime.
"As part of the UN's 53-nation Asian group, Syria's candidacy would be virtually assured of victory due to the prevalent system of fixed slates, whereby regional groups orchestrate uncontested elections, naming only as many candidates as allotted seats," Neuer wrote on the UN Watch website.
"That's how non-democracies like China, Cuba, Russia and Saudi Arabia won their current seats, and how Pakistan and Venezuela are about to do the same."
The Paris meeting came to a close after the defection of a top general dealt a blow to Syrian government forces.
Western officials say Syrian Brig.-Gen. Manaf Tlass, a member of the elite Republican Guards and a son of a former defence minister, abandoned Assad's regime. It was the highest profile departure in 16 months of bloodshed that activists say has killed more than 14,000 people.
There was more violence Friday as anti-regime activists said Assad's forces killed at least 25 people, arrested scores more and torched dozens of homes while seizing a northern city from rebels.
Baird said Canada would contribute another $1 million toward humanitarian aid in Syria.
— With files from The Associated Press