International talks to find a way to resolve the conflict in Syria "remain challenging" and a deal may not be reached, a senior US official said on Saturday.
Major powers have been meeting in Geneva to discuss the issue and decide the fate of the country's President Bashar al-Assad.
The official said: "Discussions remain challenging. We're continuing to work on this today, but we need a plan that is strong and credible. So we may get there, we may not."
The official spoke after international mediator Kofi Annan met separately with US Secretary of State Hillary Clinton and Russian Foreign Minister Sergei Lavrov at the start of talks in Geneva which had been intended to forge a consensus plan for Syria.
The United States and its European and Arab allies see no way ahead while power remains in the hands of Assad. However, Moscow, a long-time ally of the Syrian strongman and an opponent of what it sees as foreign meddling in domestic affairs, objects to any solution imposed on Syria from outside.
The UN estimates at least 10,000 people have been killed as Assad's forces have tried to suppress the uprising against him.