By Stephanie van den Berg

THE HAGUE (Reuters) - The arrest of former Philippines President Rodrigo Duterte has given a timely boost to the International Criminal Court, which is facing U.S. sanctions and an investigation into alleged sexual misconduct by its chief prosecutor.

Duterte was detained in Manila on an ICC warrant and flown to The Hague, where he was taken into custody on Wednesday. He is awaiting an initial court appearance in ICC custody.

"It's a game changer right now in terms of the court's credibility," said Danya Chaikel, a representative of human rights organisation FIDH.

Prosecutors at the world's permanent war crimes court accuse the 79-year-old of forming and arming death squads held responsible for the killing of thousands of drug users and dealers during his rule.

Duterte, speaking in a video posted on social media, said he took full responsibility for the "war on drugs". Prosecutors at the ICC intend to charge him with crimes against humanity for systematic attacks on the civilian population.

However, the court has been criticised for its slow trials and relatively low number of convictions. In two decades, prosecutors won six convictions for the court's core crimes, all of them former African militia leaders.

It has no police force and relies on its 125 member states to detain fugitives. Its top suspects remain at large. 

Outstanding arrest warrants for political and military leaders include Russian President Vladimir Putin, Israeli Prime Minister Benjamin Netanyahu and former Sudanese president Omar al-Bashir. 

Israel and Russia have denied that their forces have committed atrocities in Gaza and Ukraine respectively and say the ICC has no jurisdiction over them. When Bashir's warrant was issued, Sudan slammed the ICC as a neo-colonialist court.

The arrest warrant issued for Netanyahu prompted U.S. President Donald Trump to reinstate sanctions through an executive order. So far, it has targeted lead prosecutor Karim Khan, but it could be widened to include anyone who assists the court's investigations.

KHAN UNDER INVESTIGATION

On top of the external threats, which the court's president says put it in jeopardy, Khan is the subject of a U.N.-led investigation into alleged sexual misconduct, which he denies.

"It's been a couple of really rough months," said Iva Vukusic, assistant professor of international history at Utrecht university. "This is an opportunity for the court to be active, be involved, and show that there's a reason for it to exist," she added.

ICC spokesperson Fadi El Abdallah would not comment other than to say that all cases are important to the ICC.

Duterte is set to become the first Asian former head of state to appear before the court. 

"This is huge," said international lawyer Priya Pillai, who leads the Asia Justice Coalition.

Pillai also pointed to the significance of the charges of crimes against humanity against Duterte for killings of civilians in a government crackdown, not a military conflict.

For the victims of Duterte's "war on drugs", his arrival in The Hague feels like an answer to their prayers, said Gilbert Andres, a lawyer who represents families of victims of drug-related killings. 

"They did not really expect that justice can still be served to a powerful man such as Rodrigo Duterte," he told Reuters.

The war on drugs was the signature campaign platform that swept Duterte to power in 2016. During his six years in office, 6,200 suspects were killed during anti-drug operations, by the police's count. Activists say the real toll was far greater.

Duterte has long insisted he instructed police to kill only in self-defence and has always defended the crackdown, repeatedly telling his supporters he was ready to "rot in jail" if it meant ridding the Philippines of illicit drugs.

A court of last resort, the ICC was founded to prosecute war crimes, crimes against humanity, genocide and aggression when its member states fail to act.

Police investigations in the Philippines have moved at a snail's pace, by the administration's own admission, and neither Duterte nor any of his top police commanders have been charged with crimes locally.

In a video statement late on Wednesday, Khan acknowledged criticism of international legal institutions.

"Many say that international law is not as strong as we want, I agree with that. But I also repeatedly emphasise that international law is not as weak as we think," he said.

(Reporting by Stephanie van den Berg; Editing by Anthony Deutsch and Alison Williams)