A failed raid to capture terrorist Isnilon Totoni Hapilon in the Philippines has sparked a month-long siege in one of the archipelago's many islands.

Filipino forces have been fighting against Maute militants in the town of Marawi since 23 May, when the fighters began attacking the town and kidnapping civilians in retaliation to the army's failed attempt to capture Hapilon.

The Maute militants are affiliated to both the Isis terror group and Hapilon's Abu Sayyaf group (ASG), the latter considered to be the most violent extremist group in southern Philippines, an impoverished area mainly inhabited by Muslims in an otherwise Christian-majority country.

Who are the Abu Sayyaf militants?

Islamist south east Asia
Armed Philippine Marine soldiers escort two suspected members of Abu Sayyaf terrorist group Nasser Hapilon (L) and Hamid Abdul Baser after they were captured in a raid AFP

ASG was founded in the 1990s after separating from the Moro National Liberation Front (MNLF), one of the major movements advocating for self-determination and autonomy for Muslims in the Philippines. The split was caused as ASG wants to create a separate Islamic state.

The group, whose name means "bearer of the sword" in Arabic, has had several leaders. Its founder, Abdurajik Abubakar Janjalani, was an Islamic preacher with alleged connections to Osama Bin Laden, founder of the Al-Qaida terrorist organisation.

Al-Qaida is believed to have provided the group with financial assistance and training following its inception.

Janjalani was killed during clashes with Filipino police in 1998 and replaced by his brother, Abdurajak. When also Abdurajak died during fighting with authorities in 2006, the group split into several factions, which sometimes compete against each other.

ASG is known for carrying out kidnappings for ransom and attacks against both civilians and the army.

The size of the group is estimated to be of between 200 and 400 members. Although it is small in size, ASG has been dubbed as one of the most violent groups in the area.

It made headlines worldwide in 2001 when it abducted 20 civilians – 17 Filipinos and three Americans – from the Dos Palmas Resort on a private island in the southwestern Philippines. The hostage crisis ended with the death of at least five abductees and several Filipino soldiers. One year later, the District of Columbia charged some ASG members – including Hapilon – for their involvement in the hostage crisis.

ASG is responsible for one of the worst terror attacks in the Philippines, which killed at least 116 people when a ferry was bombed in 2004.

Kidnappings for ransom

Since 2014, it has become increasingly notorious for using kidnapping-for-ransom as a way to fund itself, according to the Site Intelligence Group.

"ASG has grown dramatically within the past few years, likely due to its funds acquired through kidnapping, and it remains one of the most violent terrorist groups in the Philippines. Its notorious ransom and beheading videos most often feature foreign nationals, in which demands are made upon the victims' respective governments," said SITE.

In March, the Philippines army recovered the remains of German hostage Jurgen Kantner, who had been reportedly beheaded by ASG militants, after their $600,000 (£469,189) ransom demand was not met.

A similar incident occurred in June 2016, when the group decapitated Canadian hostage Robert Hall after a ransom deadline expired.

Isnilon Totoni Hapilon and Isis

Islamic State
Leader of Abu Sayyaf Filipino terrorist organisation, Isnilon Totoni Hapilon, is seen in a black gown declaring his allegiance to Isis. YouTube

Hapilon, one of the most wanted terrorists in both the Philippines and the US, is also one of the most prominent leaders of ASG. His involvement with the jihadist group has been traced as far back as 1997.

The US Department of State is offering up to $5 m (£3.9 m) for information leading to Hapilon's capture or conviction.

Since ASG pledged allegiance to Isis in 2014, it has become unofficially known as the "Islamic State of Iraq and the Levant – Philippines Province". Hapilon has been dubbed as the "Emir of the Islamic State forces in the Philippines" .

After taking office in 2016, Philippines President Rodrigo Duterte said he would confront ASG.

"There will be a time that I will have to confront the Abu Sayyaf. The kidnapping must stop," Duterte said after meeting a Filipino woman who was freed after nine months of captivity.

The Filipino army said on 13 June it believed Hapilon was still hiding in Marawi's Barangay Lilod Madaya area, where at least 13 Filipino soldiers were killed during clashes with militants earlier in June.

Violence in Marawi has killed dozens and displaced thousands, prompting President Rodrigo Duterte to declare martial law on the island of Mindanao, where Marawi is located.