A history of violence
Set up in the early 1990s with al-Qaeda money, Abu Sayyaf's original mission was to create an Islamic state in western Mindanao, home to most of the 4 million Muslims in the Philippines. Abu Sayyaf's original name was Harakatul al-Islamiya, or Islamic movement.
Its founder was Abdurajik Abubakar Janjalani, an Islamic preacher and a veteran of the Afghan anti-Soviet war of the '80s. Most of Abu Sayyaf's original members first fought under the Moro National Liberation Front (MNLF), the first Filipino group to take up arms in pursuit of a 'Bangsamoro' (Nation for the Moro, as local Muslims are collectively known).
The MNLF settled for autonomy in 1976 and signed a final peace deal with Manila in 1996.
The group's first major attack was a raid on Ipil town in April 1995 that resulted in more than 50 killed, the looting of all banks and the burning of almost all buildings in the town centre.
Janjalani was killed in a clash with the Philippine National Police in 1998. His death, and the severance of the financial link with al-Qaeda, saw the group lose its ideological focus and turn to kidnappings, murders and robberies. In May 2001 Abu Sayyaf kidnapped 20 people at a Philippine resort and demanded ransom payments. One of the hostages was beheaded and two were killed. In August 2002 the group kidnapped six Filipino Jehovah's Witnesses and beheaded two of them.
But in 2003, Janjalani's brother, Khadaffy, became the group's leader. He tightened the link with Jemaah Islamiah (Southeast Asia's largest terror group) and tried to steer Abu Sayyaf back to Islamic terrorism.
Abu Sayyaf has been blamed for the country's worst terrorist attack, the bombing of a passenger ship in February 2004 that killed more than 100 people.
Intelligence reports say the group is several hundred strong.
Last August, backed by the US, Manila sent 7,000 troops to the Sulu Archipelago in an offensive aimed at eliminating the Abu Sayyaf leadership.