A United Nations human rights investigator says there are signs of mounting opposition within the Philippines to President Rodrigo Duterte's war on drugs, with police operations on hold and the Church getting critical of the campaign.

Agnes Callamard, the UN Special Rapporteur on extrajudicial, summary or arbitrary executions, however, said the thousands of killings in the campaign had given rise to a sense of impunity, which could lead to increased lawlessness and violence.

More than 7,600 people, mostly drug users and small-time dealers, have been killed since Duterte took office on June 30 last year, about a third of them in police operations. Callamard said she knew of only four court cases seeking justice for the victims.

"The difference between the number of reported killings and the number of court cases is unbelievable," she told Reuters in Bangkok. "It's very unusual for that degree of impunity to remain restricted to one kind of crime or one type of community."

Spokesmen for Duterte could not immediately be reached for comment.

The war on drugs has been a signature policy of Duterte, who remains popular in opinion polls.

But Callamard, a human rights expert from France who took up the UN post in August, said opposition to the drug war was increasing and had reached a "tipping point".

"There is an increasing awareness on the part of the Filipino people that the war on drugs could hurt them," she said. "The surveys that are being done indicate support for the president...but critique the war on drugs."

One of the Philippines' top polling agencies, Social Weather Stations, said after a survey of 1,500 people in early December that most were satisfied with Duterte's rule. But 78 per cent said they were worried that they or someone they knew would be a victim of an extra-judicial killing.

In a series of reports last year, Reuters showed that the police had a 97 per cent kill rate in their drug operations, the strongest proof yet that police were summarily shooting drug suspects.

Both the government and police have strenuously denied that extra-judicial killings have taken place.

The Church in the Philippines, Asia's largest Catholic nation, had been a muted critic of the campaign but slammed it earlier this month for creating a "reign of terror" among the poor.

The bloodshed had also generated growing unease and criticism from Philippine civil society groups and media, Callamard said.

Her remarks come as Duterte and his police chief Ronald Dela Rosa face intense criticism for the October kidnap and killing of a South Korean businessmen by anti-narcotics officers inside national police headquarters.

He was arrested for drug offences that his wife said was an official cover for kidnap for ransom.

The case, which came to light in January, prompted Dela Rosa to announce the suspension of anti-drug operations to purge the police force of what he termed "rogue cops." Duterte has however vowed to maintain his anti-drugs campaign until his term ends in 2022.

Callamard said real opposition to the drugs war would come from within the Philippines rather than international bodies such as the International Criminal Court (ICC).

In October, ICC Prosecutor Fatou Bensouda warned the Hague-based tribunal could prosecute if the killings were "committed as part of a widespread or systematic attack against a civilian population".

Duterte has threatened to withdraw from the ICC, calling it "useless", and said in a November speech: "You scare me that you will jail me? International Criminal Court? Bullshit."

(Reuters)

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Opposition against President Duterte's war on drugs mounting: UN investigator - WION