Philippine police are suspending their anti-narcotics operation until they have cleansed their ranks of "scalawags", the chief of the force said on Monday, following the killing of a South Korean businessmen by rogue officers.

The police campaign, dubbed "Oplan Double Barrel", which also includes "Tokhang", in which police go house to house knocking on doors in search of drug suspects, has claimed the lives of more than 2,000 suspected drug users and pushers.

"To all the rogue cops, beware! We no longer have a war on drugs. We now have a war on scalawags," Philippine National Police (PNP) Director-General Ronald dela Rosa told a news conference.

Dela Rosa's pronouncement came a few hours after he said he would disband anti-drugs units following the kidnapping and killing of businessman Jee Ick-joo in the national police headquarters in October.

The president and his police chief "should categorically give the order to end the killings," said Senator Leila De Lima, President Rodrigo Duterte's most outspoken critic.

The dismantling of the anti-drug units meant "they are aware that the very men involved in anti-drug operations...are involved in illegal activities under the guise of the so-called war on drugs," she told ANC television.

But Duterte vowed on Sunday to forge ahead with his war on drugs until the last day of his term.

"We will cleanse our ranks...then maybe after that, we can resume our war on drugs. The president told us to clean the organisation first," Dela Rosa said.

"I don't know how long it will take to cleanse the PNP. But with each and every one of us cooperating, helping each other, maybe in a month, we can do it."

The anti-drug campaign has caused alarm in the West and rights groups accuse Duterte of turning a blind eye to a wave of alleged extrajudicial killings by police, mostly of low-level peddlers. Police deny this, claiming self-defence.

Duterte said police officials who had been the subject of internal investigations should be reassigned to work in conflict zones.

Fighting drugs and crime was the key platform of Duterte's election campaign, during which he promised to eradicate illicit drugs within six months. His term ends in 2022.

He said he underestimated the depth of problem, and on Sunday promised the crackdown would continue until the end of his six-year presidency, and criticism would not stop him.