President Gloria Arroyo has described the group as "a money-crazed gang of criminals" without any ideology.
They call themselves the "Sword of God" in Arabic, but the Abu Sayyaf rebel group in the Philippines could hardly be seen as holy.
Beheadings, kidnappings, hefty ransoms and weapons characterise this band of armed rebels based in the country's Muslim south.
President Gloria Arroyo has described the group as "a money-crazed gang of criminals" without any ideology.
They have done little in real terms to advance their much-touted goal of an independent Muslim state, aside from bringing media attention to the area's long-running separatist conflict.
The group shot to international fame in April 2000 after kidnapping 21 people, including foreign tourists, from a Malaysian resort and taking them to a jungle hideout on Jolo island.
They were eventually released after mediation by a Libyan envoy and the reported payment of a hefty $20m ransom, which according to the military, eventually served to arm, as well as embolden, the rebels.
The episode provided lessons for President Arroyo, who has refused to negotiate with the group, despite making peace overtures to other insurgents.
She launched an all-out military offensive in response to the latest hostage crisis, in which tourists, including three Americans, were seized from a resort in Palawan island.
She has also placed a $2m bounty on the heads of the Abu Sayyaf.
Beheadings
The rebels have frequently threatened to kill their hostages - which have included children in the past - if their demands are not met.
These are no idle threats - last year, the group beheaded two teachers taken hostage on Basilan island
During last year's high-profile hostage crisis, the rebels reportedly demanded about $1m for each Western hostage.
There were reports that Jolo island was flooded with crisp, new high-denomination bills after the hostages' release.
Some of the money reportedly went into buying bazookas, mortars and Uzi assault rifles from underground Asian arms bazaars, with some rebel leaders even posing with their new weapons for newspaper photographers.
The rebels now appear to have some superior equipment to the military - according to reports, the navy's patrol boats cannot keep up with the rebel speedboats.
Factional fighting
At its peak last year, the Abu Sayyaf had about 4,000 members, with many locals drawn to the group's wealth.
But according to the military, that figure has since dwindled to 1,000 members, mostly based in the rebel heartland of Jolo.
Since September, they have come under repeated attacks by the military, which have killed hundreds of rebels.
A lack of unity may also pose a threat. Military officials have said the group has several factions, each with their own warlords, followers and territories in the Sulu islands, which have sometimes squabbled.
Commander Robot and Mujib Susukan lead one group.
But the group involved in the Palawan hostages is led by Abu Sabaya and Khadaffy Janjalani, whose Islamic preacher brother Abdurajak founded Abu Sayyaf in the early 1990s, naming it after a mujahedin fighter in Afghanistan.
Pope plot
Once the "country's most wanted man", Abdurajak was killed during clashes in 1998.
He reportedly had ties to Libya, where he had studied, and links with international extremists, such as Saudi exile Osama bin Laden.
From the start, the group was involved in armed resistance, which is almost a way of life in the south, where some insurgent groups have warring for three decades for a Muslim state, independent of the predominantly Catholic country.
Kidnapping for ransom is also common in the country.
But the group turned more ominous after attacking a Christian southern town and burning the centre.
Abu Sayyaf was also implicated in a 1995 plot to assassinate visiting Pope John Paul II.