The Philippine defence ministry has asked President Rodrigo Duterte to issue an order for the military to play a role in his "war on drugs", including granting troops powers to arrest "scalawag" police.

The ministry on Wednesday asked Duterte to formalise remarks he made in a speech to army generals the day before, when he said he wanted their help in the wide crackdown on drug-users and drug dealers alike, as well as to detain members of a police force Duterte said was "corrupt to the core".

The ministry asked for "an official order regarding this presidential directive to serve as a legal basis for our troops to follow".

"By the same token, the president's verbal directive to arrest 'scalawag cops' should also be covered by a formal order," the ministry said in a statement.

Duterte's police chief instructed the Philippine National Police (PNP) on Monday to suspend their anti-drugs operations after the killing of a South Korean businessman by rogue drug-squad police. Duterte was infuriated and embarrassed by the incident, which he said had "international implications".

Duterte's suggestion that the military should step in to fill the void left by police marks a stunning change of tack by the former city mayor, who had steadfastly supported the police amid allegations from human rights groups and some politicians of widespread abuses of power.

READ MORE: Rodrigo Duterte - The President's Report Card

The Philippine Drugs Enforcement Agency, a body a fraction of the size of the PNP, took over the lead in the crackdown, but Duterte suggested he may need the military's help.

Rights group Amnesty International said in a report on Wednesday that police prosecuting the "war on drugs" had behaved like the criminal underworld they are supposed to be suppressing, taking payments for killings and delivering bodies to funeral homes.

It said the wave of thousands of drugs-related killings in a matter of months - as part of Duterte's campaign - appeared to be "systematic, planned and organised" by authorities, and could constitute crimes against humanity.

The president's office has yet to comment on the report.

More than 7,600 people have been killed since Duterte launched his war, with at least 2,500 killings by police forces and many of the remaining deaths attributed to vigilantes and turf wars.

It is not immediately clear what role the military might play in the campaign.

The volatile president has threatened several times to declare martial law to help the crackdown, but has thus far ruled it out. He has made no suggestion in the past week of invoking military rule.

Duterte's spokesman on Tuesday said the president was fully aware of police corruption when he gave police the lead in the crackdown, but the scale of the narcotics problem was "so big" that "he had no other choice".

The Drugs Enforcement Agency said leading the crackdown without the police would be a challenge, but it could handle it.

"We can enlist the help of other agencies and other stakeholders and in fact our director general has been in several meetings with the AFP already," said the agency's spokesman, Derrick Carreon, referring to the Armed Forces of the Philippines (AFP).

"We will be deploying teams in key areas where it would be easier for them to respond. It will be more challenging ... that is why we are engaging other stakeholders."

Senator Leila De Lima, a staunch critic of Duterte, denounced the idea of involving the military.

"The solution is to stop the killings, and not call out the AFP to do the killings that the PNP has supposedly ceased to perform," she said in a statement.