MANILA (AFP) - A 14-year-old boy whose body was found with dozens of stab wounds is the latest atrocity in Philippine President Rodrigo Duterte's drug war, critics said on Wednesday (Sept 6) as outrage built at a trio of teen killings.

The National Bureau of Investigation announced it would probe the murder of the boy, who politicians and rights groups said appeared to be the third teenager murdered as part of Mr Duterte's drug war in the past month.

Filipinos have mostly backed Mr Duterte's drug war even as critics warned the thousands of people killed in the crackdown may amount to a crime against humanity. But the deaths of the three boys have triggered rare street protests and highlighted concerns about alleged police abuse.

"Under President Duterte's bloody war on drugs, there is a pattern of killing young and poor people, particularly innocent minors. There is a policy to kill," opposition Senator Risa Hontiveros said as she voiced outrage at the death of Reynaldo de Guzman.

Fellow opposition Senator Francis Pangilinan said de Guzman was among more than 30 boys who had been killed in Mr Duterte's drug war.

"The daily killings are not the solution to illegal drugs," Mr Pangilinan added.

Police said they found de Guzman's body on Wednesday in a creek about 70km from his home. His face was wrapped in packing tape and his body bore 30 stab wounds, they said.

He went missing from a suburb east of Manila 20 days ago with neighbour Carl Angelo Arnaiz, 19, who was controversially shot dead on the same night by Manila police. The police accused him of robbing a taxi driver.

The boys' killings followed the shooting death in Manila of another teenager, Kian delos Santos, during a police anti-drug raid on Aug 16.

Government investigators had said murder charges should be filed against eight police officers for the alleged murders of Arnaiz and delos Santos.

Justice Secretary Vitaliano Aguirre on Thursday also ordered an investigation into the third boy's killing.

Mr Duterte won last year's elections after promising an unprecedented crackdown to eradicate drugs in which tens of thousands of people would be killed.

Since then, police have reported killing 3,800 people in anti-drug operations, with thousands of other people murdered in unexplained circumstances.

Mr Duterte has said he would be "happy to slaughter" three million Filipino drug addicts, and that he would pardon police if they were found guilty of murder for killing in his drug war.

But he has denied ever inciting police or vigilantes to commit mass murder.