"I Fatou Bensouda, solemnly undertake that I will perform my duties and exercise my powers as prosecutor of the International Criminal Court, honourably, faithfully impartially and conscientiously," she said at a ceremony in The Hague.

The 51-year-old Bensouda, the first woman and African to head the team of prosecutors at the tribunal, had served as outgoing prosecutor Luis Moreno-Ocampo's number two since 2004.

Known as the public face of the ICC, Moreno-Ocampo stepped down on Friday after nine years as chief prosecutor at the court, which started work in 2003.

Bensouda was elected by the 121 state parties which have signed up to the Rome Statute -- the International Criminal Court's founding document.

She takes the helm of the world's first permanent court to try those accused of genocide, war crimes and crimes against humanity, investigating 15 cases in seven countries, all of them African.

ICC judges have issued 20 arrest warrants and nine summonses but only six suspects have been arrested so far and only one has been convicted -- Congolese militia leader Thomas Lubanga who used child soldiers in the 2002-3 conflict in the northeastern Democratic Republic of Congo.

Those still on the run include Sudan's President Omar al-Bashir and Ugandan Lord's Resistance Army rebel leader Joseph Kony.

Those arrested include Ivory Coast's toppled leader Laurent Gbagbo, who is awaiting a hearing to see if he will face trial on charges relating to violence that killed 3,000 people after the 2010 election.