MANILA, December 21, 2009 (AFP) - The Philippine government bears some blame for a political massacre of 57 people last month owing to its policy of cultivating warlords in the strife-torn south, a conflict prevention group said Monday.

The gruesome killings on Mindanao island were not just the product of a feud between warring political clans, the Brussels-based International Crisis Group said in a report.

It was also due "to Manila's deliberate nurturing of a ruthless warlord in exchange for votes", it said.

"To call it a feud is to diminish the Arroyo administration's role in allowing a local despot to indulge his greed and ambition, including through building up a private army in the name of fighting insurgents," said Sidney Jones, the group's senior adviser in Asia.

President Gloria Arroyo's administration has blamed the powerful Ampatuan clan for the massacre, which targeted female relatives of a rival family and about 30 journalists travelling in a convoy in Maguindanao province.

Members of the Ampatuan clan were once close allies of Arroyo, but were kicked out of the ruling coalition after the massacre.

Arroyo had supported the clan's rise to power, and used its private army in her anti-insurgency campaign against Muslim separatists on Mindanao.

In exchange, the Ampatuans allegedly ensured that Arroyo and her candidates swept the provincial voting in 2004 national elections.

The ICG said that political patronage by Manila, "most notably Arroyo", had allowed the Ampatuans "to amass great wealth and unchecked power, including the possession of a private arsenal with mortars, rocket launchers and state-of-the-art assault rifles".

Police say the clan patriarch's son Andal Ampatuan Jr. personally led the massacre, which they say was aimed at stopping a political rival from running against him for the post of Maguindanao governor in elections next May.

Ampatuan Jr. was arrested days after the killings and charged with multiple counts of murder.

His father and other clan members have been charged with rebellion, after they were rounded up when Arroyo placed their provincial stronghold under nine days of martial law.

The ICG said Arroyo must now ensure that those behind the massacre are quickly convicted and that her government must disarm private armies.

"One of the weakest aspects of the Philippine justice system is slowness," it noted.