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.

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.

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

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