MANILA • Philippine police murdered a town mayor while he was helpless in a jail cell, justice department investigators have said, contradicting claims by the accused officers and President Rodrigo Duterte that he was killed in a gun battle.

The accusations by the National Bureau of Investigation (NBI) on Tuesday deepened concerns that police are carrying out summary executions as part of Mr Duterte's controversial war on crime, which has claimed more than 5,100 lives in just over five months.

The NBI said police shot dead mayor Rolando Espinosa and cellmate Raul Yap last month as they were defenceless in a provincial jail cell.

"After conducting an exhaustive investigation of the incidents surrounding the killing of mayor Espinosa and Yap, the NBI concluded that the testimonies of several witnesses had disputed the claim of an alleged shootout between the (police) operatives and inmates mayor Espinosa and Raul Yap, but (it was) a 'rub out'," the NBI said in a statement. "Rub out" is a local expression referring to police killing a suspect and then claiming he died in a gun battle.

"The pieces of evidence, both testimonial and the forensic evidence, all agree. We believe we have a very strong case," NBI deputy director Ferdinand Lavin told reporters.

Mr Lavin said the NBI had recommended murder and perjury charges against 24 officers for their role in the alleged killings and subsequent lies. The justice department will now decide whether to file the murder charges.

The accused police officers claim they fired in self-defence at the pair when they went into the cell before dawn to carry out a search warrant.

The police allege that Espinosa, who was in jail after being arrested in October on drug and gun possession charges, had a firearm and methamphetamine in the cell.

Lawmakers, media groups and human rights advocates have ridiculed this version, asking why police had to carry out a search in a jail cell at night and why CCTV footage of the event has disappeared.

They also asked how a man in jail could have a gun and drugs, and why he would shoot at police knowing he was outnumbered.

However, Mr Duterte, who has pledged never to let a policeman go to jail for waging his war on crime, has repeatedly defended the officers. Mr Duterte had accused Espinosa, who was mayor of Albuera in the eastern province of Leyte, of being a drug lord. He gave the police "shoot-to-kill" orders if Espinosa did not surrender, prompting the mayor to turn himself in.

In a speech on Monday, he defended the police who killed Espinosa and said he believed their version of events. "Do not force me to believe the theories and assumptions, even with the witnesses, that the mayor was killed (illegally) in the prison," he said.

Mr Duterte last month signalled he was happy Espinosa was dead. "You have here a guy, a government employee, using his office and money of government, cooking (illegal drugs) and destroying the lives of so many millions of Filipinos. So what is there for me to say about it?" he said.

Mr Duterte's police chief initially stood down the police officer in charge of the raid pending an investigation, but the President reinstated him.

AGENCE FRANCE-PRESSE