The Philippines justice ministry will review thousands of killings in Philippine President Rodrigo Duterte's war on drugs, its chief said, after releasing the details of a first batch of cases that it said pointed to abuses by police.

The Department of Justice (DOJ) released details on Wednesday of 52 killings by police in the five-year-old campaign that challenges the official police narrative that claims all of the thousands of victims were drug dealers who had resisted arrest.

The Philippines has come under pressure from the United Nations to investigate allegations of the systematic murder of drug suspects, and the International Criminal Court recently announced it would investigate Duterte's bloody campaign.

Asked if the Philippines would expand its investigation into the war on drugs, Justice Secretary Menardo Guevarra said: "Time and resources permitting, the DOJ will review these thousands of other cases, too."

The release of details mark a rare admission by the state that abuses may have taken place in the anti-drugs campaign.

U.N. human rights chief Michelle Bachelet had urged the Philippines this month to publicize its findings on the 52 cases so that its work can be evaluated.

In several cases the DOJ probed, those killed had no traces of gunpowder on their hands, or they did not have a gun at all, the department said on Wednesday.

It also said police had used excessive force, shot suspects at close range, and relevant medical and police records were missing. The cases would undergo further investigation and case buildup for the possible filing of criminal charges against officers, it said.

Officially, police have killed more than 6,000 people in the crackdown, but activists say many thousands more were killed, execution style, by mysterious gunmen. Police have denied involvement in those deaths.