US Vice President Joe Biden said on Saturday the US and its partners were pushing to help strengthen the Syrian opposition, insisting President Bashar al-Assad was a "tyrant" and must go.
Biden was speaking on the second day of security talks in the German city of Munich, where he is later due to hold talks on the Syria conflict with Russian Foreign Minister Sergei Lavrov and Syrian opposition chief Moaz al-Khatib.
He said it was "no secret" that Russia and the United States have "serious differences" on issues such as Syria, where conflict has been raging since March 2011.
The United States has provided "more than $50 million in non-lethal assistance to the Syrian opposition and are working alongside our partners to help them become more inclusive and cohesive," he said.
But he added: "We are under no illusions, the days ahead will continue to be very difficult but the opposition continues to grow stronger".
"President (Barack) Obama and I, and nearly all of our partners and allies, are convinced that President Assad, a tyrant, hellbent on clinging to power, is no longer fit to lead the Syrian people and he must go," Biden said.
"We can all agree... on the increasingly desperate plight of the Syrian people and the responsibility of the international community to address that plight."
Lavrov meanwhile called for another meeting of the Syrian action group led by UN-Arab League envoy Lakhdar Brahimi in a bid to find an accord on a transition for the country and said he believed progress was possible.
Russia backs a transition plan that was agreed in Geneva in June but never implemented. The deal calls for power to be handed to an interim government but offers no clear guidance about Assad's future role.
The United States and its allies back the opposition stance that there can be no talks with Assad.
"There must be immediate implementation of what was agreed in Geneva," Lavrov said. "Talks about misinterpretation... of the Geneva meeting, it is not serious. There is nothing to be interpreted. It is bluntly stated on paper."
On Friday, Brahimi called for a "clear decision" from the UN Security Council to set the agenda for a peaceful solution.
"One has to have some kind of hope, but having hope doesn't mean being starry-eyed. Frankly now as we speak I am much more conscious of the difficulties, of the country being broken day after day than I am of a solution," Brahimi told the Munich talks.