The Philippines should begin investigations into its President Rodrigo Duterte’s claims that he killed three people while he was the mayor of Davao, the United Nations High Commissioner for Human Rights Zeid Ra’ad al-Hussein said on Tuesday. All of the killings during his crackdown on drug trafficking should also be probed, al-Hussein added.

The UN official said the killings would “clearly constitute murder”. “It should be unthinkable for any functioning judicial system not to launch investigative and judicial proceedings when someone has openly admitted being a killer,” he said, according to the New York Times. Al-Hussein said the killings Duterte described violated international laws such as equal legal protection and innocence util proven guilty.

At least 6,000 people have been killed in the since the government began its drive against drugs, including several deaths carried out by vigilante groups on motorcycles. At a business event on December 14, Duterte had said of his alleged murders, “In Davao I used to do it personally. Just to show to the guys [the police] that if I can do it why can’t you... I was really looking for a confrontation so I could kill.” Nearly 1,000 people were killed during Duterte’s tenure as the mayor of Davao from 1988 to 2013.