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The violence has also cast doubt on the durability of a recent cease-fire between the government and insurgents from the Moro Islamic Liberation Front (MILF), since members of the latter are suspected to have reinforced Abu Sayyaf.
On Saturday, President Gloria Macapagal-Arroyo called for the "annihilation" of Abu Sayyaf on the island of Basilan (click here for a map of the region), where the clashes took place, according to the Inquirer, a Philippines daily.
The Philippine Star added that the government would conduct a probe of the day-long firefight on Basilan.
The Associated Press (AP) reported that 400 government troops engaged about 150 Abu Sayyaf militants at one of their main camps last Wednesday. A unit of marine reinforcements took heavy casualties when it encountered a group of fleeing militants, according to a commander quoted by the AP.
The AP said that a MILF spokesman had confirmed that 10 MILF members were among the dead. But the spokesman said "they were not with the Abu Sayyaf and only happened to be in the area when the fighting erupted."
The incident underscores the sometimes blurry lines between the insurgent group and Abu Sayyaf.
The Al Qaeda-linked Abu Sayyaf is considered a terrorist organization by the Philippines, the US, and other governments. It's the main target of counterterror operations assisted by a few hundred US soldiers and advisers.
The MILF has been struggling for decades for greater autonomy and self-determination for Muslims in the southern Philippines. In recent years they've engaged in on-and-off peace talks with the government. The US does not consider the MILF a terrorist group.
The MILF officially denies links to terror groups and flatly denies engaging in terrorist attacks on civilians. But the International Crisis Group has detailed links between radical MILF elements, Abu Sayyaf, and the Indonesia-based terrorist group Jemaah Islamiyah.
In an editorial Monday, the Inquirer noted that the Philippines military had notified the MILF of its plan to attack the Abu Sayyaf base – a precaution taken to avoid an inadvertent clash with MILF forces. It suggested that the MILF tipped off the Abu Sayyaf to the raid, and then joined in on Abu Sayyaf's side.