MANILA: Philippine President Rodrigo Duterte announced Thursday the military would take a leading role in his deadly drug war, while vowing to kill more traffickers and addicts.

"I'm taking in the AFP (Armed Forces of the Philippines) and raising the issue of drugs as a national security threat so that I will call on all the armed forces to assist," Duterte said as he promised to kill more "son of a bitch" drug addicts.

His comments were the first following a report from Amnesty International that the killings in the drug war, in which more than 6,500 people have died in seven months, may amount to crimes against humanity.

They were also the clearest signal of Duterte's plans for the drug war, after he admitted this week the police force that had initially led the campaign was "corrupt to the core" and said they would no longer be allowed to take part.

The 71-year-old former state prosecutor won presidential elections last year after promising during the campaign to eradicate drugs in society within six months by killing tens of thousands of people.

However Duterte has had to sideline the police after a series of scandals emerged over the past month in which police were caught committing murder, kidnapping, extortion and robbery using the drug war as cover.

In one of the highest-profile cases, anti-drug officers kidnapped a South Korean businessman then murdered him inside the national police headquarters as part of an extortion racket, according to an official investigation.

Gone 'crackers'

Amnesty on Wednesday accused police of systemic human rights abuses in the drug war, including shooting dead defenceless people, fabricating evidence, paying assassins to murder drug addicts and stealing from those they killed.

It also said police were being paid by their superiors to kill. Amnesty said it documented victims as young as eight years old.

"The police are behaving like the criminal underworld that they are supposed to be enforcing the law against," Amnesty said as it warned that the International Criminal Court may need to investigate possible crimes against humanity.

However Duterte was unrepentant on Thursday as he launched a profanity-laced tirade against his critics and rejected charges of human rights abuses.

He gave a lengthy explanation of the problems for people who used the highly addictive methamphetamine known locally as shabu and may have gone "crackers".

"And you bleed for those son(s) of a bitch," he said, adding that roughly 3,000 had been killed so far.

"I will kill more. If only to get rid of drugs."

Police have reported killing 2,555 people in the drug war, while nearly 4,000 others have died in unexplained circumstances, according to official figures.

As he announced plans for using soldiers in the drug crackdown, Duterte remarked how he only had "limited warm bodies" and discussed the pros and cons of imposing "martial law" to fight drugs.

"If it's really needed to preserve the country, maybe. But it's not the right thing to do at this moment, as you can see," Duterte said.

The defence department, which oversees the military, has asked Duterte's executive secretary for a written official order to serve as legal basis for the military's participation in the drug war, ministry spokesman Arsenio Andolong told AFP.

Phelim Kine, Asia deputy director of Human Rights Watch, warned that using the military for policing forces anywhere "heightens the risk of unnecessary or excesive force and inappropriate military tactics".

"There is also a deeply rooted culture of impunity for military abuses in the Philippines," with only one soldier convicted of an extrajudicial killing since 2001, Kine said.