A self-confessed assassin who testified to being in a "death squad" under Philippine President Rodrigo Duterte will soon file a case at the International Criminal Court accusing the president of crimes against humanity, his lawyer said Tuesday.

More than 8,000 people have died since Duterte took office in June and unleashed a bloody war on drugs, a third in raids and sting operations by police who say they acted in self-defense.

Duterte and the police have denied links to the other killings, many of which rights groups say bear the same hallmarks as hundreds of suspicious deaths of criminals in Davao City during the 22 years Duterte was its mayor.