The United Nations human rights commissioner has urged the Philippines to launch a murder investigation into President Rodrigo Duterte's claim that he killed three people as mayor of Davao City.

On 14 December, the bombastic president told reporters how he had roamed the streets "looking for trouble" and personally killed criminals in his war on drugs.

More than 6,000 people have also died since Duterte assumed the presidency in July.

In a statement, Zeid Ra'ad Al Hussein, UN High Commissioner for Human Rights, said: "The killings committed by President Duterte, by his own admission, at a time when he was a mayor, 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."

Zeid added: "The killings described by President Duterte also violate international law, including the right to life, freedom from violence and force, due process and fair trial, equal protection before the law, and innocence until proven guilty."

Zeid also said that Duterte's repeated calls for the police, military and the general public to engage in a "war on drugs" and bring people in "dead or alive", has emboldened an environment of alarming impunity and violence.

He added that repeated statements indicating immunity for police officers who engaged in human rights violations in the line of duty were "a direct violation of all democratic safeguards that have been established to uphold justice and the rule of law".

Duterte's allies in Congress said the president is immune from any lawsuit and cannot be investigated for his actions before he assumed office. He can only face an inquiry after he is removed from power, they claimed.

"In fact, then-Justice Secretary Leila de Lima had investigated such cases and came up with no actionable offences," said Congressman Rodolfo Farinas, according to Reuters.

"The president could only be impeached for acts or omissions committed as president."