The Philippine police will investigate former president Rodrigo Duterte's claims that he assembled a "death squad" to control crime when he was mayor of the city of Davao.

This week, the 79-year-old pointed at former police officers present at a Senate inquiry into his deadly war on drugs as leaders of that death squad, the existence of which the former president and his allies had for years denied.

Mr Duterte later said gangsters, not police, made up the hit team.

Philippine National Police (PNP) spokesperson Ms Jean Fajardo said the names mentioned in the hearing would be used as leads and the basis for its investigation.

She said the PNP chief has instructed officers to extract unsolved cases of deaths related to drugs, including those that may be linked to the so-called death squad.

Human rights groups documented about 1,400 suspicious killings in Davao during the 22 years Mr Duterte was mayor.

Critics said the war on drugs Mr Duterte unleashed when he became president in 2016 bore the same hallmarks, but with a significantly greater death toll. 

Human rights groups estimate more than 20,000 were killed during under Duterte's administration, which triggered an International Criminal Court investigation as a possible crime against humanity.

When Mr Duterte was president, two men including a former police officer told the Senate they were part of an alleged hit squad in Davao that killed at Mr Duterte's behest, but legislators at the time found no proof. Duterte's aides dismissed the claims as fabricated.

Senator Ronaldo dela Rosa, a former police chief and the lead enforcer of Duterte's nationwide crackdown, previously said the death squads were "fiction".

In a hearing on Monday, he downplayed Mr Duterte's latest remarks, saying they should be taken as a joke.

According to police data, more than 6,200 people died in anti-drug operations under Mr Duterte's presidency, during which police typically said they killed suspects in self-defence.

Human rights groups believe the real toll of the drugs war to be far greater, with thousands more users and small-time peddlers killed in mysterious circumstances by unknown assailants.

"[Mr] Duterte's admissions were very, very damaging. It will have a major weight in the decision of the ICC," said human rights lawyer Neri Colmenares, who also represents families of drug war victims.

Mr Duterte, who was elected on promises to wipe out crime and drugs and kill thousands of dealers, offered no apologies in the hearing, which was also attended by victims' families.

"If you let me back, I would do it all over again," he told senators.

Reuters/AP/ABC