(Adds Russian abstention, paragraph 7)

By Patrick Worsnip

UNITED NATIONS, Sept 14 (Reuters) - The U.N. Security Council on Friday extended the appointment of Carla del Ponte as prosecutor of war crimes in the former Yugoslavia until Dec. 31, when her still-unnamed successor will take over.

The outspoken Del Ponte, from Switzerland, has held the job for eight years and her term of office had been due to end on Friday. She has been appointed Swiss ambassador to Argentina starting next year.

As Yugoslavia prosecutor she has helped bring a number of suspects to court, including former Serbian President Slobodan Milosevic, who died last year before his trial could be completed.

But her high-profile search for former Bosnian Serb leader Radovan Karadzic and his military commander, Ratko Mladic, has so far failed to track them down.

Rumors have circulated for months at the United Nations her successor will be Belgium's Serge Brammertz, who currently leads the U.N. investigation into the 2005 killing of former Lebanese Prime Minister Rafik al-Hariri. His mandate in that job also expires at the end of the year.

U.N. Secretary-General Ban Ki-moon must submit to the Security Council his nominee as new prosecutor of the Hague-based International Criminal Tribunal for the Former Yugoslavia.

A Security Council resolution noted "the need to ensure a smooth transition between the departure of Ms. Carla del Ponte and the assumption of office of her successor." Russia, which is close to Serbia, abstained in the vote but the council's other 14 members voted in favor.

In a separate unanimous resolution, the Security Council reappointed Hassan Bubacar Jallow of Gambia as prosecutor of the International Criminal Tribunal for Rwanda for another four years.

The resolution urged the Arusha, Tanzania-based court, which prosecutes suspects in the 1994 genocide in Rwanda, to try to wrap up all its work in 2010.

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