U.S. Secretary of State Hillary Rodham Clinton and Russian Foreign Minister Sergey Lavrov were heading for a face-to-face showdown over Syria on Friday as major powers prepared for a weekend conference to hash out a political transition plan for the country.

On the eve of Saturday’s conference aimed at ending 16 months of brutal violence in Syria, Clinton and Lavrov were to meet in St. Petersburg in a bid to iron out deep differences over the transition plan being pushed by U.N. envoy Kofi Annan that calls for the formation of a national unity government that would oversee the drafting of a new constitution and elections.

U.S. officials are adamant that the plan will not allow Syrian President Bashar Assad to remain in power at the top of the transitional government, but Russia insists that outsiders cannot dictate the ultimate solution or the composition of the interim administration.

Annan laid out his expectations for the weekend conference in an op-ed in The Washington Post. The future government in Syria, he said “must include a government of national unity that would exercise full executive powers. This government could include members of the present government and the opposition and other groups, but those whose continued presence and participation would undermine the credibility of the transition and jeopardize stability and reconciliation would be excluded.’’

Such a proposal does not explicitly bar Assad, but the U.S. and other western powers who will participate in the conference said that is implicit.

Assad also said any future government in Syria must hold free and fair elections for a multiparty government.

Russia is Syria’s most important ally, protector and supplier of arms. Diplomatic hopes for have rested on persuading Russia to agree to a plan that would end the Assad family dynasty, which has ruled Syria for more than four decades.