UNITED NATIONS: The search has started for a replacement for Syria mediator Lakhdar Brahimi, who diplomatic sources say is planning to resign in the near future, largely out of frustration at Syrian President Bashar Assad’s plans to hold an election in June.

There are several possible candidates to replace the veteran Algerian diplomat, the sources said, among them former Tunisian Foreign Minister Kamel Morjane.

For more than a year, Brahimi has made no secret that he is contemplating stepping down from the post as the United Nations and Arab League joint special representative on Syria. Brahimi told reporters a year ago in New York that he thought about resigning every day.

Security Council diplomats say Brahimi will brief the 15-nation body on May 13 on his so far unsuccessful efforts to bring an end to the Syrian civil war, now in its fourth year.

One senior Western diplomat told reporters Wednesday that he expected Brahimi to make known his intention to resign during his stay in New York, where he was due Friday for talks with U.N. Secretary-General Ban Ki-moon. Another senior diplomat confirmed the remarks. Both spoke on condition of anonymity.

A senior U.N. official told Reuters that it was not a question of whether Brahimi would resign, but when.

“It will certainly happen soon,” the official said.

U.N. spokesman Farhan Haq declined to comment on what diplomats described as Brahimi’s imminent resignation.

“The secretary-general very much appreciates the ongoing role that Joint Special Representative Lakhdar Brahimi plays in trying to end the appalling violence in Syria,” Haq said. “We do not speculate on personnel matters.”

Brahimi has organized 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 U.N. 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 a bid by Assad to defy widespread opposition and extend his grip on power.

U.N. spokesman Stephane Dujarric said after the vote was announced that Brahimi and Ban had “warned that the holding of elections in the current circumstances amid the ongoing conflict and massive displacement will damage the political process and hamper the prospects for a political solution.”

Another Western diplomat told Reuters that “Brahimi had indicated that he would resign if the election went ahead, so we’re expecting his resignation.”

Diplomatic sources say that Tunisia’s Morjane, who was defense and then foreign minister from 2005 until the 2011 uprising led to the ouster of President Zine al-Abidine Ben Ali, is among the leading candidates to replace Brahimi.

“The Tunisian name in on there, but there are others,” a senior Western diplomat said. He did not name other candidates.

Brahimi’s predecessor, former U.N. Secretary-General Kofi Annan, resigned in frustration in August 2012. Like Brahimi, he complained that the permanent members of the U.N. Security Council could not unite behind his calls for an end to the violence and a peaceful political transition.

More than 150,000 people have been killed in the war, while some 2.5 million people have fled abroad.

Russia, supported by China, has vetoed three resolutions that would have condemned Assad’s government, threatened sanctions and called for accountability for any war crimes.

The Syrian government and Moscow, however, accuse the U.S., Britain and France of undermining peace efforts by helping Saudi Arabia, Turkey and Qatar finance and arm the rebels.