Lakhdar Brahimi has announced his resignation from his position as the UN-Arab League envoy for Syria, largely out of frustration at Syrian President Bashar al-Assad's plans to hold an election in June.
UN Secretary-General Ban Ki-moon, in a joint press conference with Brahimi in New York on Tuesday, said the decision would be effective from May 31.
Addressing the media at the UN, Brahimi expressed regret that he had been unable to help the Syrian people: "Apologies once more that we haven't been able to help [the Syrian people] as much as they deserve, as much as we should have, and also to tell them that the tragedy in their country shall be solved... they have shown incredible resilience and dignity."
"An immense majority of Syrians want peace and stability in their country and I'm sure they will get it," he added.
For more than a year, Brahimi has made no secret that he is contemplating stepping down from the post as the UN and Arab League joint special representative on Syria. Brahimi told reporters a year ago that he thought about resigning every day.
"It's not very pleasant for me. It's very sad that I leave this position and leave Syria behind in such a bad state," Brahimi told reporters after Ban announced his departure.
"Everybody who has responsibility and an influence in the situation has to remember that the question is how many more dead? How much more destruction is there going to be before Syria becomes again the Syria we have known," he said.
The conflict has killed more than 150,000 people and displaced some nine million people.
Brahimi has organised two rounds of negotiations in Geneva between Assad's government and members of the opposition seeking to oust him.
While there were no breakthroughs at those talks, diplomats and UN officials said that Brahimi had wanted to continue the Geneva process to find a negotiated solution that would end the fighting, launch a political transition and begin the process of reconciliation between the supporters and opponents of Assad.
But Syria's April 21 announcement that it will hold presidential elections on June 3 dealt a severe blow to Brahimi's efforts in Geneva, diplomats said, since the vote is widely seen as a bid by Assad to defy widespread opposition and extend his grip on power.
Ban blamed the failure of the peace effort on the warring parties, but especially the Syrian government.
He also blamed the deeply divided Security Council and countries with influence on the fighting sides. Ban pledged to keep working to achieve peace and urged all involved to rethink what they can do to bring hope to the Syrian people.
Diplomatic sources say that Tunisia's Kamel Morjane, who was the defence and then foreign minister from 2005 until the 2011 "Arab Spring" uprising led to the ouster of President Zine el-Abidine Ben Ali, was among the leading candidates to replace Brahimi.
|A look back at Brahimi's tenure as the UN-Arab League envoy for Syria
Al Jazeera's Diplomatic Editor James Bays said other names floating around included Kevin Rudd, former Australian prime minister, Javier Solana, former secretary-general of the Council of the European Union, and Sigrid Kaag, a UN diplomat who heads the mission for the destruction of Syria's chemical weapons.
The opposition Syrian National Council said it appreciated Brahimi's work to try to bring a political settlement to the conflict, and that it remained committed to a political process.
"But it is clear that the regime will not desist from its brutal military campaign and engage in a political process until it is compelled to do so," the coalition said in a statement. "That will require concerted international pressure that has so far been lacking."
Brahimi's predecessor, former UN Secretary-General Kofi Annan, resigned in frustration in August 2012. Like Brahimi, he complained that the permanent members of the UN Security Council could not unite behind his calls for an end to the violence and a peaceful political transition.