A Philippine court has sentenced three police officers to up to 49 years in jail for the murder of a 17-year-old high school student, in the first such convictions in President Rodrigo Duterte's war on drugs.

Key points: State agents have killed at least 5,000 since President Rodrigo Duterte has taken office

State agents have killed at least 5,000 since President Rodrigo Duterte has taken office Police maintain that drug users and dealers weren't killed in executions, but shootouts

Police maintain that drug users and dealers weren't killed in executions, but shootouts Mr Duterte has said he would not allow police to go to jail for killing drug users and dealers

The Caloocan City regional trial court declared the three officers guilty for the killing of Kian Lloyd delos Santos in August 2017 in a dark, trash-filled alley in a northern suburb in the capital Manila.

"A shoot first, think later attitude can never be countenanced in a civilised society," Judge Rodolfo Azucena said.

"Never has homicide or murder been a function of law enforcement.

"The public peace is never predicated on the cost of human life."

This was the first case of what human rights advocates said was an extrajudicial killing carried out by state agents in the 29-month war on drugs, which saw at least 5,000 people killed in police anti-drugs operations.

Police reject allegations the killings were executions, saying the drug peddlers and users were killed in shootouts, and that they were acting in self-defence.

The death of the school boy has stirred unprecedented public attention on what activists say are executions and systematic abuses by police backed steadfastly by Mr Duterte.

Mourners display streamers during a funeral march for Kian delos Santos. ( Reuters: Dondi Tawatao )

Mr Duterte, a firebrand leader who unleashed a ferocious war against illegal drugs after coming into office in June 2016, has more than once said he would not allow the police to go to jail for killing drug users and pushers.

But shortly after the verdict was handed down, Mr Duterte's spokesperson, Salvador Panelo said: "This is murder, there is intention to kill.

"The President would never tolerate that."

Mr Duterte's Government has repeatedly said there was no declared policy to kill drug users and pushers.

"The conviction of the three police officers for murdering Kian de los Santos is a victory for justice, but it is not enough," said Jose Manuel Diokno, chairman of the Free Legal Assistance Group (FLAG).

"The killings must stop."

FLAG has questioned the legality of the drugs war before the Philippine Supreme Court.

President Rodrigo Duterte meets with Kian's parents, Saldy delos Santos and Lorenzana delos Santos. ( Supplied: Malacanang Palace )

Mr delos Santos was found dead in an alley with a gun in his left hand.

Police said they killed him in self-defence, but his family dismissed that as a lie.

Security cameras showed the officers aggressively escorting a man matching Mr delos Santos's description in the direction of the spot where he was killed.

"Justice was served for my son," Kian's mother Lorenza delos Santos said outside the courtroom.

"We were able to prove that my son was innocent of all the accusations hurled against him."

Sorry, this video has expired Family of teenager killed in war on drugs seeks justice

Reuters