Lakhdar Brahimi has resigned as the UN and Arab League mediator for Syria on Tuesday after international efforts to find a political solution to the three-year-old civil war failed.
"I thought we'd be able to deliver but because of divisions we have not been able to make any progress in the course of three years," UN Secretary General Ban Ki-moon.
Ban and Brahimi appeared together to announce the resignation, which takes effect at the end of the month.
Ban said he had accepted the resignation "with great regret" and said a replacement for the Algerian diplomat and veteran peace negotiator had not yet been chosen.
Brahimi, who was to meet later in the day to brief senior envoys on the 15-member UN Security Council, said he was "very sad ... (to) leave Syria behind in such a bad state."
Signup to our newsletter and follow us on Facebook and Twitter!
Ban, however, praised Brahimi, saying he had faced "almost impossible odds" but had "persevered with great patience."
He blamed the Syrian government of President Bashar al-Assad and the opposition for the failure.
"We were here to help them," he said. "I am urging them again to think of their own future this is their country, their future."
Brahimi was named international mediator of the Syria conflict on August 17, 2012, replacing former UN secretary general Kofi Annan, who also failed to obtain a ceasefire.
The 80-year-old managed to get the two sides to meet face to face for the first time early this year in Geneva, only to see the negotiations collapse in the second round.
He himself had said he had not expected much from the talks, which were organized under pressure from Washington, which supports some rebel factions, and Moscow, Assad's main ally.
He had also said Syria's presidential elections scheduled for June 3 would shatter his diplomatic efforts. The rebels would never return to the negotiating table if Assad were re-elected, as seems likely.