Syrian peace talks planned for January are the "best opportunity" to form a transitional government, US Secretary of State John Kerry has said.
He was speaking shortly after UN Secretary General Ban Ki-moon announced that Syria's government and opposition would meet on 22 January in Geneva.
The UN, US and Russia have been trying for months to get both sides to agree a political solution to the conflict.
But there have been disputes over who should attend the talks in Switzerland.
The Syrian opposition is divided over which groups should represent it in Geneva. But the opposition insists President Bashar al-Assad must resign.