Richard Haass, director of the New York-based Council on Foreign Relations and a former US envoy to Northern Ireland from 2001 to 2003, chaired the multiparty talks.
The disputes that Haass has been trying to bring hinge on Catholic opposition to Protestant marches, the most fundamental trigger point for Northern Ireland violence; the contested rights of both sides to fly their preferred British and Irish flags, an argument that sparked Protestant street blockades and clashes with police; and the question of how to honour and bring justice for the 3,700 dead from a nearly 45-year-old conflict.
"The work done on flags is quite disappointing by any measure," he said. "But the other two areas have been, I believe, quite impressive and I believe it would be a real shame not to be able to turn that work into a reality."
Haass, who now will go back to the US, insisted the process is "still alive" and said he might return to Belfast next weekend if reactions to a revised proposal indicate progress could be made.
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"Will explore if a last push justified," he added on Twitter. "Hope so, as all would gain from pact."
Ulster Unionist Party leader Mike Nesbitt expressed optimism as he left the talks, saying the talks "didn't get there tonight" but he that "by next Tuesday, who knows, maybe we will have an agreement."
Alliance party deputy leader Naomi Long was less positive, asserting that while the process may not be dead it is on "life support.