People watch on TV the screening of the start of the International Criminal Court trial of former child soldier-turned-warlord Dominic Ongwen in Lukodi, Uganda on Tuesday. Photo: AFP

Britain's Karim Khan starts Wednesday as prosecutor of the International Criminal Court (ICC) with a daunting caseload including a probe into Israel and the Palestinians, the most politically fraught file in the tribunal's history.

Khan replaces Gambia's Fatou Bensouda, who dramatically extended the ICC's reach during her nine-year tenure but suffered a series of high-profile failures, including the acquittal of former Cote d'Ivoire president Laurent Gbagbo.

The 51-year-old Khan, who was elected by ICC member nations in February to become just the third prosecutor so far of the world's only permanent war crimes court, will be sworn in at a ceremony in The Hague at 0900 GMT.

He will take a public oath of office declaring: "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."

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

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

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

"The ICC is in a crucial phase, it has faced criticism for not being as effective as states have wished," said Carsten Stahn, international criminal law professor at the University of Leiden in the Netherlands, who interviewed Khan in 2015 for his course students.

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

The ICC's investigation into the 2014 Israel-Palestinian conflict in Gaza promises to be particularly contentious.

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

"The ICC might be seen as an actor which is not fully impartial in the context... It's going to be very difficult to navigate the difficult expectations with regard to the engagement of the ICC in this case."

"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.