GENEVA – The United States and Russia failed on Friday to bridge differences over a plan to ease Syrian President Bashar Assad out of power, end violence and create a new government, setting the stage for the potential collapse of a key multinational conference that was to have endorsed the proposal.

On the eve of Saturday’s conference, U.S. Secretary of State Hillary Rodham Clinton and Russian Foreign Minister Sergey Lavrov met one-on-one for about an hour in St. Petersburg, Russia, but could not reach agreement on key elements of U.N. envoy Kofi Annan’s proposed plan for a Syrian political transition, officials said.

A senior U.S. official traveling with Clinton said areas of “difference and difficulty” remain and was not optimistic that the gathering in Geneva would produce agreement. “We may get there tomorrow, we may not,” the official told reporters as Clinton left Russia for Switzerland.

The official said Clinton and Lavrov would try to resolve differences in Geneva out of respect for Annan, the former U.N. chief whose efforts to end the Syrian crisis have thus far fallen short.

The inconclusive results of the Clinton-Lavrov meeting may presage the unraveling of Annan’s plan to end 16 months of brutal violence in Syria by creating a national unity government to oversee the drafting of a new constitution and elections.

The U.S. and its allies attending the conference are adamant that the plan will not allow Assad to remain in power as part of the transitional government, but Russia insists that outsiders cannot dictate the composition of the interim administration or the ultimate solution to the crisis.

“(We) agreed to look for an agreement that will bring us closer based on a clear understanding of what’s written in the Annan plan that (all) sides in Syria need an incentive for a national dialogue,” Lavrov said after meeting Clinton, according to the Interfax news agency.

“But it’s only up to the Syrians to make agreements on what the Syrian state will be like, who will hold (government) jobs and positions,” he said. Lavrov predicted the meeting had a “good chance” of finding a way forward. “But I am not saying that we will agree on every dot.”

But failing to agree on every dot may well be the plan’s undoing, particularly if Russia refuses to except the implicit demand that Assad leave power.