Heavily armed officers of the police Anti-Illegal Drugs Group burst into the home of South Korean businessman Jee Ick-joo in Angeles City, 80 kilometres north of Manila, as the bodies of drugs suspects were piling up in funeral parlours across the island nation.

The police had a fake arrest warrant and accused Mr Jee of involvement in drugs, according to a Department of Justice investigation.

Mr Jee was dragged to a police vehicle, where he offered a bribe to the police, even though he was not a drug user or seller, the investigation found.

The police wanted more.

Mr Jee was driven to the Philippine National Police headquarters at Camp Crame, in Quezon City, which was used as a detention centre during the dictatorship of Ferdinand Marcos and was a rallying point for the popular uprising against him in 1986.

Just hours after arriving in the sprawling grounds on October 18 last year, Mr Jee was strangled to death, the investigation found.

Over the following two weeks the police kidnappers demanded and received a $US100,000 ransom from Mr Jee's family, whose members were led to believe he was still alive.

Human rights groups cite the case as an indicator of the breakdown of the rule of law under Mr Duterte's rule, which has been condemned by the United Nations.

The police confirmed this week that the number of Filipinos killed in the crackdown that began when Mr Duterte took office on June 30 has now surpassed 7000 – an average of more than 30 deaths a day.

This includes 2503 accused drug users or dealers supposedly killed by police while they were resisting arrest and a further 3603 killed by so-called "unidentified gunmen".

The kidnapping and murder of Mr Jee has deepened suspicions of the involvement of police in what human rights groups describe as extra-judicial killings amounting to crimes against humanity.

"Police justify those 2503 killings, saying the victims resisted arrest and shot at police officers," said Phelim Kine, Human Rights Watch's Asian Division deputy director.

"But police have not provided further evidence that officers acted in self-defence.

"There are allegations that death squads composed of plainclothes police personnel are behind some of the unidentified killings."

Mr Duterte, a foul-mouthed former provincial mayor, has shrugged off international condemnation of the killings and scuttled a Senate probe into the deaths as an army of internet trolls harass, intimidate and threaten critics who question the crackdown's logic or legality.

The president's popularity still hovers above 60 per cent in the majority Catholic country where drugs have ravaged impoverished communities.

Two policemen have been arrested over the murder of Mr Jee, one of only a few cases where police have been called to account for deaths under the guise of the crackdown.

Mr Duterte's office has said there will be no cover-up or whitewash of Mr Jee's case although comments he has made in the past have been interpreted as giving police impunity in drugs cases, including telling them to shoot suspects on sight if they refuse to give themselves up.

"We are outraged by the abduction and slaying," a presidential spokesman said.

Philippine National Police chief Ronald dela Rosa, a close associate of Mr Duterte, has dismissed calls for him to resign over the killing.

"They want me to resign? Then you can tell the president to fire me. But we still have a mission, there's a mission we must accomplish," he told reporters.

"We'll finish first the war on drugs. When the Philippines is drugs-free, then I can resign."