The United States has strongly backed a move by Syria's opposition chief to open a dialogue with President Bashar al-Assad to end 22 months of warfare that has cost tens of thousands of lives.

Ahmed Moaz al-Khatib, the leader of the Syrian Opposition Coalition, told pan-Arab television channel Al-Jazeera on Monday that "the ball is now in the government's court. They will either say yes or no".

He was following up on his surprise announcement last week that he was ready for talks with the government, but subject to conditions, including the release of 160,000 detainees.