SPO3 Arthur Lacanas answers questions during a press conference regarding his and President Duterte's alleged involvement in the Davao Death Squad, held at the Senate in Pasay City on Monday. Voltaire Domingo, NPPA

MANILA - Philippine senators allied with President Rodrigo Duterte sought on Wednesday to block a retired policeman from testifying that Duterte had once operated a "death squad", dismissing the former policeman as a liar.

The retired policeman, Arturo Lascanas, told a news conference on Monday he had been a ringleader of a death squad run by Duterte when he was mayor of the southern city of Davao.

His confession contradicted testimony he had given to a Senate inquiry last year, during which he said no such death squad existed.

But on Monday, Lascanas said the death squad started operating when Duterte became mayor of Davao in 1988, and members got cash and orders from Duterte to kill criminals.

Duterte has not commented on the allegations though he has denied similar accusations in the past. His legal counsel, Salvador Panelo, said the allegations were a "continuing fabrication" and "another false narration".

A prominent critic of Duterte, Senator Antonio Trillanes, has called for Lascanas to give testimony to a Senate inquiry, but Duterte's allies rejected that.

"This is not the proper forum for this man who has lied," Senator Richard Gordon told the assembly.

"No one should be allowed to trifle with the senate insofar as the truth in concerned."

The Senate committee on justice, which Gordon heads, last year found no proof that a Davao death squad existed or that killings that had taken place when Duterte was mayor, and since he became president in June, were state sponsored.

Human rights groups have documented about 1,400 suspicious killings in Davao while Duterte was mayor and critics say the war on drugs he unleashed when he became president has the same hallmarks.

More than 7,700 people have been killed since June, about 2,500 in what police say are shootouts during raids and sting operations. Most of the rest are under investigation and activists believe many were extrajudicial killings.

Philippine boxing champion Manny Pacquiao, also a senator and a Duterte ally, said allowing Lascanas to testify again would set a precedent that "a witness can just lie and just change his testimony".

But some opposition members disagreed.

Senator Grace Poe said Lascanas' allegations were "serious" and the senate must be given the "opportunity to test his credibility."

Senator Paolo Benigno Aquino, a cousin of former President Benigno Aquino, said in a television interview that Lascanas had risked his life and freedom to go public with his story, so it was the legislature's duty to hear what he had to say.

Lascanas' allegations are similar to some made by a self-confessed hit man, Edgar Matobato, to a Senate investigation last year about killings in Davao.

Duterte dismissed Matobato's allegations. (Reporting by Karen Lema; Editing by Robert Birsel)