The United States on Tuesday declined to set a timeline for a peace conference on Syria, further dimming prospects for fresh talks which diplomats had initially hoped to stage in June or July.
US State Department spokesman Patrick Ventrell said Washington wanted the Geneva meeting to take place "as soon as possible."
"We want to have this as soon as possible, we've been clear about that," Ventrell said. "Clearly, the situation on the ground, clearly, the regime's continued avoidance of this real discussion, are serious impediments."
"I'm just not going to put a timeline on it, but we think that it's an important process, and we'll continue to pursue it."
UN peace envoy to Syria Lakhdar Brahimi said in Geneva on Tuesday the conference would probably not take place in July as hoped for.
"Frankly, I doubt that the conference will take place in July," he told reporters in Geneva ahead of a second meeting with US and Russian diplomats.
Brahimi stressed the urgency of holding the conference, dubbed Geneva 2, which is meant to follow up on an initial meeting in the Swiss city last year that produced a never-implemented transition plan for Syria.
While all sides have repeatedly said Geneva 2 should take place as quickly as possible, it is looking ever more unlikely that the conference will happen any time soon.
The 27-month conflict in Syria has claimed more than 93,000 lives, according to the United Nations.