Hopes for a breakthrough in a push to end Afghanistan's gruelling conflict suffered a major setback Friday after a key summit between the Taliban and Afghan officials was indefinitely postponed.

The so-called intra-Afghan dialogue, due to take place in Doha this weekend, fell apart at the last minute in a row over the large number of delegates Kabul wanted to send.

The collapse comes at a critical time and amid continued bloodshed. The Taliban now control or influence about half of Afghanistan and 3,804 civilians were killed there last year, according to a UN tally.

Washington, which is leading an effort to end the war, signalled its disappointment and urged both sides to return to the table, though organisers gave no hint about when the conference might be rescheduled.

Sultan Barakat, who heads the group that was to host the event, said in a statement the postponement was "necessary to build further consensus as to who should participate".

"Clearly the moment is not yet right," added Barakat, the director of the Center for Conflict and Humanitarian Studies.

President Ashraf Ghani's administration had on Tuesday announced a list of 250 people from all walks of Afghan life, including government figures, who it wanted to send to Doha.