Here's a list of key fighting groups inside Syria:

— SYRIAN GOVERNMENT — Despite major defections early in the conflict and the loss of significant territory to rebels, the Syrian military remains a potent force against an outgunned opposition. President Bashar Assad's inner circle has largely remained cohesive and united, avoiding high-level defections that would sap its strength. The military has successfully exploited its greatest advantage, its uncontested airpower, to pound opposition-held areas and sustain far-flung bases holding out in rebel territory. Assad has bolstered his overstretched military over the past year with the creation of the National Defense Force, a pro-government militia that draws heavily from Syria's minority communities and reportedly receives training from Iran.

— HEZBOLLAH: The Lebanese Shiite militant group has sent its gunmen to fight alongside Assad's forces, providing a significant boost to the government's overstretched military. Hezbollah leader Sheik Hassan Nasrallah has suggested he would do everything it takes to save the Syrian government, which has been a patron and ally of the militant group for decades. Hezbollah's critics say the group's armed intervention in Syria has stoked sectarian tensions at home and needlessly dragged Lebanon into the maelstrom next door. Hezbollah's deep involvement in Syria underlines the regional sectarian aspect of the conflict, in which an Iranian-backed Shiite axis faces off against Sunnis supported by Gulf Arabs in a proxy war extending into Lebanon and Iraq.

— ISLAMIC STATE OF IRAQ AND THE LEVANT: Al-Qaida's longtime affiliate in Iraq. In the spring of 2013, the Islamic State moved aggressively into Syria, establishing a major presence particularly in the opposition-held north. Syrian activists say the group is largely composed of foreign fighters, and is the most ruthless opposition outfit on the battlefield. It has not limited its efforts to fighting the government alone, but has opened fronts against more moderate rebel groups, as well as Syria's Kurdish minority. Over time, the group alienated many in the territory under its control by employing brutal tactics to impose its strict interpretation of Islamic law and silence its critics. Those tactics include kidnapping, torture and beheadings. In early January, an array of Islamist and more moderate rebel groups began attacking the Islamic State across seven northern provinces in a bloody spate of infighting that has killed more than 1,000 people. Still, the group does cooperate with other rebel factions for specific operations.

— JABHAT AL-NUSRA: An Islamist extremist group affiliated with al-Qaida. Jabhat al-Nusra, or the Nusra Front, is one of the most powerful rebel factions on the battlefield, but has been eclipsed to a degree by the Islamic State. In contrast to that group, activists say the Nusra Front is primarily composed of Syrians, and has shown a pragmatic streak and ability to compromise with other rebel groups that the Islamic State has not. The U.S. has designated the Nusra Front a terrorist organization. The group has claimed responsibility for many of the deadliest suicide bombings targeting regime and military facilities. The presence of Islamic extremists among the rebels is one reason the West has not equipped the Syrian opposition with sophisticated weapons, such as anti-aircraft missiles.

— ISLAMIC FRONT: An alliance of seven powerful conservative and ultraconservative rebel groups that merged in late November. Analysts estimate the number of fighters in the group could number as high as 45,000. 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 have publicly criticized the Islamic State of Iraq and the Levant for its brutal tactics, and the Islamic Front was among the rebel factions that have battled the al-Qaida-linked group this month. The Islamic Front rejects the Geneva conference, and has said it will not abide by any agreement reached at the talks.

— SUPREME MILITARY COUNCIL: Syria's more moderate rebel units, known together as the Free Syrian Army, regrouped more than a year ago under a unified rebel command called the Supreme Military Council and headed by Gen. Salim Idris. Idris spent more than 30 years in the Syrian military and is seen as a secular-minded moderate. The Supreme Military Council and its FSA brigades have been eclipsed over the past year by more conservative groups like the Islamic Front (which contains former FSA outfits) as well as extremist factions like the Nusra Front and the Islamic State. 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.