The al-Aqsa Martyrs' Brigades is an armed Palestinian group associated with Yasser Arafat's Fatah organisation.
The group has attacked soldiers, settlers and civilians
The group, which emerged shortly after the outbreak of the current Palestinian intifada, has carried out operations against Israeli soldiers and settlers in the West Bank and Gaza, and suicide attacks on civilians inside Israel.
The brigade is neither officially recognised nor openly backed by Mr Arafat and Fatah, though brigade members tend also to belong to Fatah, the Palestinian leader's political faction.
Although other militant groups, as well as Fatah, agreed to a temporary suspension of attacks on Israeli and Jewish targets in June 2003, the al-Aqsa Martyrs' Brigades has not.
Israeli forces have often targeted the group's leaders.
Raed Karmi, its then leader in the West Bank, was killed in an explosion in January 2002.
Israel alleges that Marwan Barghouti, the Fatah leader in the West Bank, is also the head of the al-Aqsa Martyrs' Brigades.
Mr Barghouti was captured by Israeli forces in April 2002 and is on trial.
Analysts say that unlike Hamas, the al-Aqsa Martyrs' Brigades does not want an Islamic state, but uses Islam to inspire its struggle for an independent Palestine.
The brigade is said to have grown out of Fatah's need to be seen to be putting up some kind of resistance to Israeli raids into land that was meant to be under Palestinian control.
Israel accuses Marwan Barghouti of leading al-Aqsa Brigade
By allowing the brigade to operate, in the hope that it would make life difficult for the Israeli army, Mr Arafat may have made both a military and political miscalculation.
One of the Israeli responses to attacks by the al-Aqsa Martyrs' Brigades and other groups were the heavy attacks against the Palestinian Authority during 2002, which saw much of the authority's infrastructure destroyed.
Mr Arafat's tacit backing for the brigade has also allowed Israeli officials to paint him as backing terrorism.