Philippine police are suspending their controversial war on drugs after the "corrupt" police force has been "cleansed".

Police Chief Ronald dela Rosa said on Monday that anti-drug units would be dissolved.

It comes after the murder of a South Korean businessman inside police headquarters. He had been kidnapped and killed by anti-drug police.

More than 7,000 people have been killed since the crackdown on drugs began.

The death toll and President Rodrigo Duterte's hardline stance against drugs have attracted intense criticism from human rights groups and Western countries, although the president continues to enjoy a high level of support among Filipinos.

Speaking on Monday, Mr Dela Rosa said Mr Duterte "told us to clean the organisation first".

"We will cleanse our ranks... then maybe after that, we can resume our war on drugs."

Mr Duterte has made tackling drug use in the Philippines a central part of his presidency.

He had initially promised to eradicate the problem by December, then extended the deadline to March this year.

But he told reporters at a press conference late on Sunday: "I will extend it to the last day of my term... March no longer applies." Mr Duterte's term ends in 2022.

He said he had underestimated the depth of the drug problem.

Senator Leila De Lima, Mr Duterte's most vocal critic, said the president and the police chief "should categorically give the order to end the killings".

She said the dismantling of the police anti-narcotics operation 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.

'Corrupt to the core'

Mr Duterte also railed against the police force on Sunday and vowed to "cleanse" it, in response to the high-profile killing of South Korean businessman Jee Ick-joo in October.

Jee Ick-joo was seized in October from his home in Angeles city, near Manila, under the pretence of a drug raid, the Department of Justice said. After strangling him, his killers pretended he was still alive in order to collect a ransom from his family.

"You policemen are the most corrupt. You are corrupt to the core. It's in your system," Mr Duterte said, adding that he thought up to 40% of policemen were used to corruption.

Mr Duterte had sanctioned extra-judicial killings previously, saying he would pardon policemen who kill criminals and civilians in the line of duty.

"When I said I'll protect the police, I'll protect the police. But I won't protect lying," he said.