By Michelle Nichols and Tom Miles
UNITED NATIONS/GENEVA (Reuters) - Veteran United Nations official Staffan de Mistura, a former U.N. special envoy to Afghanistan and Iraq, will replace Lakhdar Brahimi as the international mediator seeking an end to Syria's civil war, diplomats said on Wednesday.
U.N. Secretary-General Ban Ki-moon will speak to the media later on Wednesday, said spokesman Stephane Durjarric. Ban will announce the appointment of de Mistura, a dual citizen of Italy and Sweden, said diplomats, speaking on condition of anonymity.
The move comes amid worsening violence as Islamist militants seized swathes of Syria and Iraq and after Syrian President Bashar al-Assad was re-elected in a June 3 poll described by Ban as a blow to international efforts to end to the conflict.
Algerian diplomat Brahimi stepped down on May 31, frustrated by global deadlock over how to end the three year civil war in Syria. He had long threatened to quit, just as his predecessor - former U.N. Secretary-General Kofi Annan - did in 2012.
Annan resigned after six months as the U.N. and Arab League joint special representative on Syria, accusing the U.N. Security Council of failing to unite behind efforts to end the fighting that has now killed more than 150,000 people.
Russia, supported by China, has vetoed four Security Council resolutions threatening action against its ally Syria.
The United Nations says 10.8 million people in Syria need help, of which 4.7 million are in hard-to-reach areas, while three million others have fled the conflict.
De Mistura, has worked with the United Nations in Somalia, the Middle East, the Balkans, Nepal, Iraq and Afghanistan during the past three decades. He was most recently the U.N. special envoy in Afghanistan in 2010-2011.
It was not immediately clear if he would be a joint U.N. and Arab League envoy. Ban Ki-moon met Nabil el-Araby, head of the Cairo-based Arab League, in June to discuss whether a new appointee would still be a joint representative.
In a keynote speech on Syria on June 20, Ban said the warring parties in Syria had systematically blocked tireless efforts by "two of the world's most brilliant and experienced diplomats" - Annan and Brahimi.
"Diplomacy seems to have stopped in its tracks. The presidential election ... was a further blow to the political process," said Ban at the Asia Society in New York.
He said that the new mediator would not "be able to wave a magic wand."
"The special envoy will strive to advance the U.N.'s protection agenda, and will work with the parties and their regional backers in a search for new elements on which to build some hope of a political process," Ban said.
Regional states also have a special responsibility to help end the war, Ban said, welcoming contacts between Iran and Saudi Arabia that he hoped would build confidence and reverse a "destructive competition" in Syria, Iraq and elsewhere.
(Additional reporting by Stephanie Nebehay in Geneva; Editing by Ralph Boulton)