Philippines President Rodrigo Duterte has vowed to pursue rogue police after three controversial killings of youths. Picture: AP Photo/Bullit Marquez

Manila - Philippine President Rodrigo Duterte vowed on Wednesday to pursue cases against erring police officers after three controversial killings of youths.

Duterte said he has ordered the Department of Justice to investigate the killing of 19-year old Carl Angelo Arnaiz after he met his parents on Wednesday.

"I have always said we will protect the soldiers and the policemen, no doubt about it. But always, there has to be the element of the performance of duty and you do not kill defenceless persons," Duterte said in speech before social security administration employees.

"I'm sorry but I will pursue the cases against the police and if need be, they should go to jail."

Arnaiz, who police accused of robbery, died of five gunshot wounds and an autopsy report showed he was shot while on his back. But police have said Arnaiz opened fire on them and paraffin tests showed gunpowder burns on his right hand.

Duterte unleashed an anti-narcotics crackdown the day he took office on June 30 last year after a convincing win in an election in which he campaigned heavily on a promise to use deadly force to wipe out crime and drugs.

According to police records, more than 3 800 people have died in police operations since July last year. Police say most were killed resisting arrest.

The manner of Arnaiz's killing was similar to that of high school student Kian Loyd delos Santos, who was shot dead in the same city where Arnaiz was killed, according to the Public Attorney's Office.

The death of delos Santos stirred rare public outrage about Duterte’s ferocious war on drugs, but the Philippine leader remains widely popular.

The Department of Justice on Wednesday also ordered the National Bureau of Investigation to look into the death of 14-year-old Reynaldo De Guzman.

Arnaiz's parents said De Guzman was last seen with their son before they disappeared.

De Guzman was found dead on Wednesday with 31 stab wounds, Senator Risa Hontiveros said in a statement. Media said his body was found about 100 km (60 miles) north of the capital.

"I hope this removes any doubt and settles with finality any discussion of whether or not there is a pattern and policy of killings under the Duterte government," Hontiveros, a staunch critic of the president, said.