A summary of some of the rebel groups fighting in Syria to overthrow President Bashar Assad

Some of the main rebel groups fighting in Syria to overthrow President Bashar Assad:

ISLAMIC STATE OF IRAQ AND THE LEVANT: A rogue al-Qaida breakaway group that first emerged as an affiliate of the international extremist network in Iraq. It moved aggressively into Syria in the spring of 2013, establishing a major presence particularly in the opposition-held north and northeast. The group is dominated by foreign fighters and has muscled in on territory held by other rebel groups, creating resentment. The group has also alienated many by employing brutal tactics — including kidnappings, torture and beheadings — in trying to impose its strict interpretation of Islamic law and silence its critics.

In early January, an array of Islamic and more moderate rebel groups began attacking the Islamic State across seven northern provinces in a bloody spate of infighting that has now killed more than 2,000 people. On Feb. 3, the Islamic State was expelled from the al-Qaida franchise, accused of refusing to listen to mediators trying to stem infighting. Islamic State fighters have also been accused of assassinating a senior al-Qaida militant in Syria who was appointed to negotiate between rebels.

NUSRA FRONT: An Islamic extremist group affiliated with al-Qaida. Nusra Front, or Jabhat al-Nusra in Arabic, is one of the most powerful rebel factions on the battlefield. In contrast to the Islamic State, activists say the Nusra Front is dominated by Syrian fighters, and shows a pragmatic streak and ability to compromise with other rebel groups. Nusra Front is engaged in a rivalry with the Islamic State but has tried to stay out of the rebel infighting for the most part. The Nusra Front is led by Abu Mohammed al-Golani and has been designated by the U.S. as a terrorist organization. The Nusra Front has claimed responsibility for many of the deadliest suicide bombings targeting regime and military facilities.

ISLAMIC FRONT: An alliance of seven powerful conservative and ultraconservative rebel groups that merged in late November. Analysts estimate as many as 45,000 fighters could be in this alliance. The Islamic Front wants to create an Islamic state in Syria, and rejects the Western-backed Syrian National Coalition and its military wing, known as the Supreme Military Council. Leaders of the Islamic Front, which includes the ultraconservative Ahrar al-Sham faction, have publicly criticized the Islamic State of Iraq and the Levant for its brutal tactics. The Islamic Front is among the chief rebel factions that have battled the al-Qaida-linked group since January.

SUPREME MILITARY COUNCIL: Syria's more moderate rebel units, known together as the Free Syrian Army. They regrouped more than a year ago under a unified rebel command called the Supreme Military Council, but have been eclipsed by groups like the Islamic Front and extremist factions. The fading fortunes of the Supreme Military Council stem in part from its inability to secure greater support, particularly the delivery of weapons, from its Western and Arab allies. The FSA was most recently thrown into disarray in February after some of its members ousted their commander, Gen. Salim Idris, and announced a new chief, Brig. Gen. Abdul-Ilah al-Bashir. Idris and some of his FSA allies rejected the ouster.

SYRIA REVOLUTIONARIES FRONT: A coalition of rebel brigades, many of whom broke away from the Free Syrian Army. It is led by Jamal Maarouf, also known as Abu Khaled. Although it's not technically part of the Free Syrian Army, it is considered aligned with the exiled Syrian opposition. Some analysts believe the Revolutionary Front was formed to create a better-organized conduit for Western allies to supply arms and money to rebels. The Revolutionary Front has been waging war against the Islamic State of Iraq and the Levant.

SOUTHERN FRONT: A coalition of rebel brigades that appears loyal to the Free Syria Army, headed by Bashar al-Zoubi. One of the first reports of the Southern Command emerged in October, but it's unclear when it was first formed. The Southern command appears to fight around the southern province of Daraa.