The opposing sides will not initially meet face-to-face in planned Syria peace talks in Geneva next week, US secretary of state John Kerry has said.
It was a sign that obstacles remain in the latest diplomatic push to end the conflict.
The first UN-brokered meeting in the Swiss city will be “proximity talks” in which representatives of the government and opposition will gather separately, Mr Kerry said on the sidelines of the World Economic Forum in Davos, Switzerland.
The talks are tentatively planned to take place on Monday, but diplomats say that could slip by up to a few days.
The cautious, step-by-step approach points to the delicate task faced by UN Special Envoy for Syria Staffan de Mistura.
He is eager to make concrete progress towards ending the nearly five-year-old war that has claimed more than 250,000 lives, displaced millions and given an opening to Islamic State to seize land.
The intra-Syrian talks also come against the backdrop of a string of recent battlefield victories by the government that have bolstered President Bashar Assad’s hand and plunged the rebels into disarray, raising the prospect that the Geneva talks could become moot as the situation on the ground evolves.
“You are not going to have a situation where people are sitting down at the table staring at each other or shouting at each other,” Mr Kerry said.
“You’re going to have to build some process here.”
Such talks have fallen apart before: An attempt in Montreux, Switzerland, in January and February 2014 collapsed when the Syrian delegation refused to discuss Mr Assad and branded the opposition terrorists.