A Philippine court on Thursday convicted three police officers of murdering a 17-year-old boy during a crackdown on narcotics in a slum neighbourhood in 2017.

The verdict marks the first conviction of police officers under President Rodrigo Duterte’s campaign against illicit drugs, which has left nearly 5,000 people dead since July 2016.

National police chief Director General Oscar Albayalde vowed the conviction would not hamper the campaign against illegal drugs.

“This case can serve as a reminder for the rest of our personnel to be extra diligent in fulfilling the requirements of the law as we serve and protect our citizens,” he said in a statement.

“But this will not cause us to waiver a bit in our resolve to rid this society of the menace of illegal drugs,” he added.

Judge Rodolfo Azucena sentenced each officer to up to 40 years in prison, without eligibility for parole, for the killing of Kian Lloyd Delos Santos in the suburban city of Caloocan on August 16, 2017.

But Azucena found the officers not guilty of planting illegal drugs and other evidence, such as a pistol allegedly used by Delos Santos to fight back, a key part of the officers' defence case.

The judge noted that while the court acknowledged the danger faced by law enforcers in their jobs, “the use of unnecessary force or wanton violence is not justified.”

“A shoot first, think later attitude can never be countenanced in a civilized society,” he said in his 35-page decision. “Never has homicide or murder been a function of law enforcement. The public peace is never predicated on the cost of human life.”

Delos Santos was one of three teenagers whose deaths last year triggered public outrage over Duterte’s drug war.

The three police officers were caught on security footage dragging Delos Santos to a dark alley in his neighbourhood, where he was shot in the head while kneeling down and begging for his life.

“We are very happy that the rule of law and truth have prevailed in this case,” said lawyer Persida Acosta, chief of the Public Attorney’s Office that assisted the Delos Santos family during the case.

Delos Santos’ mother, Lorenza, said Thursday she hoped the verdict would serve as a warning to other police officers not to commit abuses.

“This proves that my son was innocent and was not involved with illegal drugs,“ she added. “The police should be afraid because this case proves that they can be jailed for life if they don’t stop the bad things they are doing.”

Human Rights Watch cautiously hailed the decision, noting that Duterte once promised to pardon any law enforcer convicted in drug war killings.

“This is a triumph of justice and accountability and a warning to members of the Philippine National Police to respect due process and the rights of civilians as they do their job,” said Brad Adams, Asia director of the New York-based group.

“But at the same time that we are heartened by this, we are also wary because Duterte has promised to pardon police officers convicted in the drug war killings,” he added. “There is reason to suspect that he will keep that promise.”

But presidential spokesman Salvador Panelo hailed the ruling, noting that Duterte does not tolerate “any wrongdoing by men in uniform” nor encourage impunity.

“Never has the president said that erring government workers, much less the police, should evade responsibility for wrongful acts,” he said.