Maguindanao Massacre Witness Killed: AFP

MANILA, Philippines (Agence France-Presse) – A key witness in the trial of a powerful Muslim clan accused of orchestrating the worst political massacre in the Philippines has been shot dead, a prosecutor said Thursday.

The witness, Suwaib Upham, claimed to have taken part in the November killings of 57 people in a crime allegedly planned by his former employers, the Ampatuan clan.

"He was supposed to be one of our strongest witnesses," prosecutor Harry Roque told Agence France-Presse. "He saw, and participated in, the killings and could have directly named in court those involved."

Roque warned that Upham's killing, which he was told occurred last week in the southern province of Maguindanao, could potentially weaken the case against the Ampatuan family.

US-based Human Rights Watch also said the killing raised doubts about the government's resolve in seeing justice done in the case.

"Massacre witnesses are dying while the government sits on its hands," the group's Asia director Elaine Pearson said in a statement. "This sends the worst possible message to other witnesses thinking of coming forward."

His death comes two months after an uncle of another witness was also shot and killed, in what authorities said was part of a plan to intimidate those speaking out against the Ampatuan clan.

The clan had ruled Maguindanao with brutal efficiency for a decade prior to the massacre. They also enjoyed political ties with outgoing President Gloria Arroyo, who used the family's huge private army as a force against separatist rebels.

Six clan members are among 196 people charged over the murders, allegedly carried out to prevent a member of a rival clan from running as governor of the province. (AFP)