A former police officer has claimed that Philippine President Rodrigo Duterte used to pay him and other police officers to murder criminals.

During a news conference on Monday, Arthur Lascanas said that while he was a mayor, Duterte used to make a "liquidation squad" kill criminals and opponents.

He stressed that the reason he was speaking up was that he had a guilty conscience, especially for his role in the death of two of his brothers who were killed for being 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.

He also described several other incidents which he said had been ordered by Duterte to carry out such actions as bombing a mosque in 1993 or killing a drug lord.

In one incident he said that his group had shot a kidnapping suspect, along with his pregnant spouse, son, father-in-law, and two other people. According to human rights lawyers, Lascanas’ claims could be used to impeach Duterte.

Martin Andanar, Duterte's communications secretary, rejected all the allegations, saying that they were a "demolition job" by unnamed persons affected by Duterte's reforms and aimed at his removal from power.

"The press conference of self-confessed hitman SPO3 Arthur Lascanas is part of a protracted political drama aimed to destroy the president and to topple his administration," he added.

Human rights groups accuse Duterte of turning a blind eye to a wave of alleged extrajudicial killings by police, mostly of low-level peddlers. Police denies carrying out such killings.

Thousands of people have been killed in the campaign against drugs since it started on June 30 last year. According to police figures, about 2,000 drug suspects have been killed in security operations, while nearly 4,000 others have died in unexplained circumstances linked to the crackdown, according to media.

During his presidential election campaign in mid-2016, Duterte ran mainly on a platform of fighting drugs and crime.