The plea for an international investigation came Friday, a day after Edgar Matobato, 57, told a Senate hearings panel investigating Duterte's current campaign against drugs (pictured above) that in the 1990s, he had seen Duterte - then mayor of Davao - shoot a man dead and order assassinations.
During his testimony, Matobato said he was a member of a "Davao death squad" which had killed hundreds of Duterte opponents and suspected criminals and had even fed a man to a crocodile.
New York-based watchdog group Human Rights Watch (HRW) called on Manila to invite the United Nations to probe the claims.
"President Duterte can't be expected to investigate himself, so it is crucial that the United Nations is called in to lead such an effort," HRW Asia director Brad Adams said in a statement. "Otherwise, Filipinos may never know if the president was directly responsible for extrajudicial killings."
The US State Department has also responded to the senate testimony.
"These are serious allegations and we take them seriously - we look into them," said deputy spokesperson Mark Toner.
Doubts over testimony
A spokesman for Duterte and several of his supporters have cast doubt on the credibility of Matobato's testimony. The national police chief, Ronald Dela Rosa, has called Matobato a "false witness," and numerous lawmakers have said the accusations could not be taken seriously.
"There are many lapses in his testimonies," said Senator Panfilo Lacson. "Like in baseball, I was counting his strikeouts."
Critics of Duterte say the alleged killings in Davao during his more than two decades as mayor set a pattern that has expanded nationwide during his presidency.
That sentiment was echoed by the senator heading the panel looking into Duterte's drug war, Leila de Lima.
De Lima conceded Matobato could be wrong about some dates, but added: "My impression is that he is not lying."
House of Representatives member Edcel Lagman on Friday urged Duterte to name an independent fact-finding commission comprised of retired judges to "determine the identities of the principals and perpetrators as well as of the victims."
se/cmk (Reuters, AFP, KNA)