A retired Philippine police officer says President Rodrigo Duterte, when he was a mayor, ordered and paid him and other members of a "liquidation squad" to kill criminals and opponents.

The former policeman, Arthur Lascanas, told a news conference on Monday that he was speaking up because he was bothered by his conscience - including his role in the deaths of his two brothers, whom he ordered killed because they were drug users.

"I had my own two brothers killed. Even if I end up dead, I'm content because I've fulfilled my promise to the Lord to make a public confession," he said, breaking into tears.

READ MORE: Children and Duterte's drug war - Lessons from the past

Human rights lawyers who presented Lascanas at a news conference said his allegations could be grounds for impeaching Duterte.

Presidential Communications Secretary Martin Andanar described Lascanas' claims as "a protracted political drama" and attempted "character assassination" of Duterte orchestrated by his critics to "topple" his administration.

Duterte has denied his government backs unlawful killings of suspects under his deadly crackdown against illegal drugs, which is feared to have killed more than 7,000 mostly drug users and petty pushers since he took office in June.

The killings under the crackdown, an expansion of his anti-drug campaign when he was a longtime mayor of southern Davao city, have alarmed the United States, other Western governments, and UN rights officials.

Lascanas said the gang of hitmen were paid between $400 and $2,000 per murder, depending on the target.

"Of all the killings we did in Davao City, either we bury them or throw them into the sea. It is paid [for] by Mayor Rody Duterte," he told the news conference at the Senate in the capital Manila.

In many public speeches, Duterte has told policemen to defend themselves if drug suspects fight back and has openly threatened drug lords and dealers with death.

Lascanas's comments came after he denied to a Senate hearing last year that he had been involved in any extrajudicial killings in Davao.

He testified at the inquiry last October after he was implicated by another witness, Edgar Matobato, a former militiaman who said Duterte ordered him and others to kill criminals in gangland-style assaults that left hundreds of people dead.

Lascanas narrated several killings he said Duterte had ordered, permitted or financed as mayor of Davao, including the 1993 bombing of mosques as retaliation after Muslim rebels were blamed for the bombing of a Roman Catholic cathedral.

READ MORE: Thousands march against Duterte's war on drugs

Lascanas said he and his group shot dead a kidnapping suspect along with the man's pregnant wife, young son, father-in-law and two others with the consent of Duterte.

After his group informed Duterte about the capture of the suspected mastermind of a kidnapping in Davao, Lascanas quoted the mayor as saying: "All right, make it clean."

Another target was radio commentator Jun Pala, who was critical of Duterte. He was killed in 2003 by gunmen, who then got financial rewards from the then mayor, Lascanas said.