Syria mediator Lakhdar Brahimi, who is stepping down on May 31, has apologised to the Syrian people for failing to broker peace in the three-year civil war.
Brahimi had long threatened to quit, just as his predecessor - former UN secretary general Kofi Annan - did in 2012.
Brahimi, a veteran Algerian diplomat, briefed the 15-member Security Council for the final time on Tuesday.
"I go with a heavy heart because so little was achieved," he told the closed-door session of the council. "I once again, humbly apologise to the Syrian people."
International and regional powers have backed opposing sides in the civil war that has so far killed at least 150,000 people, with Russia and Iran supporting President Bashar al-Assad and Western powers and Gulf Arab states largely backing the rebels.
Brahimi left "a few ideas" with UN Secretary General Ban Ki-moon and the Security Council - focus on getting more aid into Syria, reduce and end the violence, end the flow of arms into Syria and help form an executive to lead Syria's transition by organising a national dialogue, conference, constitutional review and elections.
Ban said he would work on finding someone to replace Brahimi. There are several possible candidates, among them former Tunisian foreign minister Kamel Morjane.
Ban praised Brahimi's perseverance in the face of "impossible odds, a Syrian nation, Middle Eastern region and wider international community that have been hopelessly divided in their approaches to ending the conflict".
"That his efforts have not received effective support from the United Nations body that is charged with upholding peace and security, and from countries with influence on the Syrian situation, is a failure of all of us," Ban said.
The Security Council has been split on how to deal with the war. Syrian ally Russia, supported by China, has blocked three resolutions that would have condemned Assad's government, threatened sanctions and called for war crimes accountability.
That deadlock is set to be exposed again when the council votes on a French-drafted resolution to refer Syria's civil war to the International Criminal Court. French UN ambassador Gerard Araud said he hoped a vote could happen early next week.
In announcing Brahimi's departure, Ban said: "There must be accountability for the terrible crimes that have been - and are being - committed. Such crimes include the deliberate starvation of communities by preventing humanitarian access."
US Secretary of State John Kerry said Brahimi did not fail. "It is the fault of a party, Assad, who will not negotiate, who absolutely refused to negotiate at every single session," he said.