MANILA - President Rodrigo Duterte on Tuesday ordered the police to conduct anti-drug operations in the presence of journalists to prove law enforcers are not executing drug suspects.

Duterte, speaking to reporters at a ceremony at a bank, said the police should maintain their fight and confront the criminals, and they should take the media along too.

"This is what I told the police. Do not make raids now without the media," he said.

"If it's a camera, if there's a reporter, see to it that they are there in the beginning up to the end."

Duterte took office in June last year after winning an election on a vow to get tough on drugs and crime. He soon launched a "war on drugs" in which thousands of people have been killed.

Duterte and his campaign remain popular but opposition to the bloodshed, including from within the influential Catholic church, has begun to build.

Earlier on Tuesday, the Senate grilled the national police chief about the killings.

While the campaign retains support, the killing of a 17-year-old boy, Kian Loyd delos Santos, last month, after he was dragged off by plain-clothes anti-drug policemen into an alley, has stirred public outrage.

Police chief Ronald dela Rosa, testifying at a Senate inquiry at which he was asked about the boy, dismissed any suggestion there was a policy to summarily kill suspects.

"We will die for the innocent people. It's painful to say there's a policy of widespread killings," Dela Rosa, appearing to fight back emotion, told the televised hearing.

"The president never told me to kill and kill."

Police say they acted in self-defense after delos Santos opened fire on them.

Senator Risa Hontiveros, a staunch critic of Duterte, told the hearing the police "should never be used as a killing machine".

"There's a wide policy that allows the killings in the name of war on drugs," Hontiveros said.

Dela Rosa said he would step down if she could prove her accusation.

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.

Thousands of others have been killed by unknown assailants. Human Rights Watch said last month the drug-war death toll was at least 7,000.

Rights groups suspect police, or gunmen working with the encouragement of police, were behind most unexplained killings.

Police reject that, saying the murders were the result of gang wars, drug dealers killing informers, or vigilantes.

Duterte has long encouraged tough action.

Last month, after 32 people were killed in a night of police raids, he said: "Let's kill another 32 every day."

Senators also asked about the killing of 19-year-old Carl Angelo Arnaiz last month in the same neighborhood where delos Santos was shot.

Arnaiz, who the police accused of robbery, died of five gunshot wounds. A police medical examiner told the inquiry the autopsy showed he was shot while in supine position.

"There is a policy that dictates the culture of killings in our nation. There is a system to the killings, there is a method to madness," Hontiveros said.