President admitted last week to “personally” killing three men as mayor of Davao City during a gunfight in the 1980s.

The UN’s human rights chief has urged Philippine authorities to investigate President Rodrigo Duterte’s claims that he killed three people while mayor of Davao City, as well as the growing carnage in his war on drugs.

Since July when Duterte assumed the presidency, there have been 6,000 people killed in the government’s anti-drug campaign – about one third died in police operations and the rest were killed by motorcycle-riding masked men and vigilante groups.

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UN High Commissioner for Human Rights Zeid Ra’ad al-Hussein’s call for an investigation – made in a statement issued in Geneva on Tuesday – was sparked by Duterte’s recent remarks that as a town mayor in southern Davao city in 1988 he killed three suspected kidnappers in a firefight while he was backed up by three police officers.

“Philippine judicial authorities must demonstrate their commitment to upholding the rule of law and their independence from the executive by launching a murder investigation,” Zeid said.

He added that it’s “unthinkable for any functioning judicial system not to launch investigative and judicial proceedings when someone has openly admitted being a killer”.

Duterte’s allies in Congress said the president is immune from prosecution and cannot be investigated for actions allegedly committed before he assumed office.

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He can only face an inquiry after he is removed from power, they added.

“In fact, then Justice Secretary Leila de Lima had investigated such cases and came up with no actionable offences,” said Congressman Rodolfo Farinas.

“The president could only be impeached by acts or omissions committed as president.”

Duterte previously threatened to withdraw the Philippines from the United Nations for its criticism of his drug crackdown, describing the world body as futile in the face of genocidal killings in other places.