MANILA (AFP, REUTERS) - Philippine President Rodrigo Duterte on Monday (Jan 30) described the police force prosecuting his deadly crime war as "corrupt to the core", but extended his drugs crackdown until the last day of his term in 2022.

Duterte, at a press conference just after midnight, announced he intended to "cleanse" the police force, after a series of scandals in which officers were caught committing murder, extortion and robbery while using the drug war as cover. In response, the police on Monday said they have suspended the controversial anti-narcotics operations until they can rid their ranks of "scalawags".

Duterte, however, made it clear the drugs war would continue until his presidency ends and his police chief, Ronald dela Rosa, said he hoped operations could resume within a month.

"You policemen are the most corrupt. You are corrupt to the core. It's in your system," Duterte told reporters as he railed against the anti-drug officers who allegedly masterminded the murder of a South Korean businessman inside national police headquarters.

In a break from his steadfast support for the police, Duterte said nearly 40 per cent of the police force engaged in illegal activities and were as "lousy as drug lords".

His comments came seven months after he took office and immediately tasked police with being the frontline troops in his plans to wipe out the illegal drug trade that he said was threatening to turn the Philippines into a narco state.

Since then, police have reported shooting dead more than 2,500 people they have accused of being drug suspects, alleging on every occasion they had to open fire in self defence.

Duterte has been unwavering in his defence of police involved in the campaign against drugs in the face of international outrage over the death toll, and repeatedly said he would protect those accused of wrongdoing. He has, however, frequently voiced frustration at police corruption.

Human rights groups and relatives of some of the victims have alleged police frequently shoot dead defenceless people, and often plant drugs and a gun on the bullet-riddled corpse.

Nearly 4,000 other people have died in unexplained circumstances in the crackdown, according to official figures.

Many of those victims have had signs placed on them labelling them drug traffickers or users.

"To all the rogue cops, beware! We no longer have a war on drugs. We now have a war on scalawags," Dela Rosa told reporters after a flag-raising ceremony at the police headquarters where South Korean Jee Ick Joo, 53, was strangled in October. "We will cleanse our ranks."

Dela Rosa's decision to suspend the drug crackdown came a day after he announced the dismantling of all anti-drugs units due to police abuses, including the planting of evidence. The death of Jee was a disaster for the image of the police.

Jee was arrested for drug offences that his wife and lawyers said was an official cover for kidnap for ransom.

Duterte won the presidential elections largely on a law-and-order platform headlined by a vow to eliminate the illegal drug trade in three to six months.

He promised that 100,000 people would be killed and so many bodies would be dumped in Manila Bay that the fish there would grow fat from feeding on them.

Once in office Duterte extended the timeframe of the drug war until March of this year, but on Monday he said there would be no end while he was in power.

"I will extend it to the last day of my term," Duterte said. "March no longer applies."

In the Philippines, presidents are allowed to serve only a single term of six years.

Critics, however, have questioned the sincerity of Duterte's outrage against corrupt police, and why the police had been given such a free hand so far to kill in the name of the drug war if he knew so many were corrupt.

"How can a corrupt and fascist police force, where impunity is the norm, successfully stamp out criminal activities such as the illegal drug trade," said Renato Reyes, secretary general of BAYAN, a coalition of leftist activist groups.

"The body count will continue to rise and more criminals in uniform will wreck havoc on the people."

Senator Leila De Lime, Duterte's most outspoken critic, said the president had a problem with "cognitive dissonance" and it was baffling that he could so strongly denounce police yet vow no let-up in the campaign.

"I continue to worry about the state of the president's mental health," she said in a statement. "This latest incongruence between his factual assertion of a rotten police force on the one hand and his reliance on them to continue prosecuting his drug war as official government policy, has dire consequences."