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Britain's was sworn in Wednesday as the new prosecutor of the International Criminal Court, facing huge challenges, including investigations into the , Afghanistan and the Philippines.

Khan, 51, a former defence lawyer for the Hague-based tribunal, was elected by member states in February to serve a nine-year tenure at the world's only permanent war crimes court.Ìý

He has been left with a bulging case file by his predecessor Fatou Bensouda, who extended the ICC's reach so dramatically that she was hit by but also suffered a series of high-profile failures.

"The ICC is in a crucial phase, it has faced criticism for not being as effective as states have wished," Carsten Stahn, international criminal law professor at the University of Leiden in the Netherlands, told AFP.

But Stahn said Khan could bring "new momentum" and had a "window of opportunity to amend the functioning" of the court, which has also been criticised for the high salaries of its judges and its slow moving processes.

Khan took a public oath of office in a ceremony at the ICC, making him just the court's third prosecutor since it was founded in 2002 to try people for the world's worst crimes.

"I solemnly undertake that I will perform my duties and exercise my powers as Prosecutor of the International Criminal Court honourably, faithfully, impartially and conscientiously," he said.

'Face of adversity'

Khan previously led a special UN probe into crimes by the Islamic State extremist group and, more controversially, also represented late Libyan leader Moamer Kadhafi's son Seif al-Islam at the ICC.

Bensouda has left him with a full in-tray, including a probe into the Philippines war on drugs that she announced on Monday, an investigation into alleged US war crimes in Afghanistan, and the Israel-Palestinian conflict.

TheÌýICC's investigation into the 2014 Israel-Palestinian conflict in Gaza promises to be the most contentious in the court's history.

"It is a very politically charged issue," said Stahn.

In her farewell statement, Bensouda said that she had "made my decisions, with careful deliberation -- but without fear or favour. Even in the face of adversity. Even at considerable personal cost".

Bensouda said she had intended to ask for a formal investigation into crimes by government forces in Venezuela, but was prevented at the last minute after Caracas asked judges to take control of the case. Their decision is awaited.

She had also aimed to open probes into Ukraine and Nigeria but was leaving those to Khan to complete, she said.

The British lawyer will meanwhile have to contend with the outright opposition of key countries that have refused to join the ICC, including the United States, Israel, China and Russia.

'Under pressure'

Amnesty International said Khan's appointment was a chance for "revitalisation" of the ICC, but that he would face challenges in the job.

"He will be under pressure and we hope he will proceed as Fatou Bensouda in independence and without fear or favour," Matthew Cannock, head of Amnesty's Center for International Justice, told AFP.

Khan may, however, benefit from a new and less confrontational US administration, compared to the government of former president Donald Trump which imposed sanctions on Bensouda.

Bensouda had a mixed record in her tenure since 2012 even as she expanded -- some analysts say overextended -- the court's reach.

Under her leadership, former Ivory Coast president Laurent Gbagbo was cleared of crimes against humanity, while former DR Congo vice-president Jean-Pierre Bemba was acquitted on appeal, and Kenyan President Uhuru Kenyatta had charges against him dropped.

But Bensouda has recently secured high-profile convictions against Ugandan child soldier-turned-Lord's Resistance Army commander Dominic Ongwen and Congolese warlord Bosco "Terminator" Ntaganda.

She has also been credited with improving the prosecutor's office compared with her predecessor Luis Moreno-Ocampo, whose leadership was described as "autocratic" in a probe ordered by the ICC into the Kenyatta case.

"Fatou's legacy will be viewed positively. She took very courageous decisions," said Amnesty's Cannock.Ìý