Powerful Syrian insurgent groups have rejected the authority of the opposition Syrian National Coalition, damaging efforts by Western-backed political exiles to forge a moderate rebel military force on the ground.
The 13 groups include some of the strongest rebel units in Syria, and several which had been affiliated to the Supreme Military Command (SMC) of the Free Syrian Army. Reliable figures for their strength in numbers are not available.
Here is a list of the rebel groups:
NUSRA FRONT - Declared allegiance to al Qaeda leader Ayman al-Zawahri. Also known as Jabhat al-Nusra, it was designated a terrorist group by the United States and has been one of the most potent forces ranged against President Bashar al-Assad, using suicide bombers to attack military targets. Car bombs in Damascus and Aleppo that killed dozens brought it to notice from December 2011 onward. Like other rebel groups, its fighters often coordinate with other brigades for frontline operations.
AHRAR ASHAM - Leading radical Salafist group, with national reach but focus in north-west. Shares much of Nusra Front's Islamist agenda but criticizes its allegiance to al Qaeda. Aims may be more nationalist that al Qaeda's regional Islamic state.
TAWHEED BRIGADE - Led the assault on Aleppo in mid-2012, when rebels seized much of Syria's biggest city. Believed to be the largest militant group in Aleppo province. Some members favor an Islamic state in Syria but others are more secular.
SUKOUR AL-SHAM - Concentrated in the northern province of Idlib and parts of Hama, Sukour al-Sham was the driving force behind the establishment late last year of the Syrian Islamic Liberation Front (SILF), a coalition of Islamist brigades which retained links with the Free Syrian Army. Despite aligning itself with the FSA, it said that fighters on the ground would not take instructions from an outside leadership.
ISLAM BRIGADE - Another component of SILF, it is the driving force of rebel operations around Damascus. It claimed a July 2012 bombing that killed close security aides to Assad.
LIWA AL-HAQ - Formed last year in Homs and made up of Islamist units fighting in and around the central city. It is led by a former paratrooper in the Syrian armed forces.
HARAKAT FAJR ASHAM AL-ISLAMIYA - Aleppo-based Salafist group created by a three-way merger in January. Founders included Islamist political prisoners released during the uprising.
NOUR AL-DIN AL-ZINKI BATTALION - Saudi-backed, Aleppo-based Islamist unit. Its members were seen in a recent video showing new weapons they had acquired, including anti-tank missiles.
FURQAN BRIGADE (AL QUNEITRA) - A branch of the Damascus-based Islamist Furqan Brigade, operating in the southwest of Syria near the Israeli-occupied Golan Heights.
LIWA AL-ANSAR - Fighting to the west of Aleppo and in Idlib. After it seized a military post from Assad's forces in March, it thanked the SMC for military and logistical support.
19TH DIVISION - Rebel unit operating in Aleppo, including Khan al-Assal. Closely linked to the Ansar Brigade. Its Facebook page identifies it as part of the Free Syrian Army.
TAJAMU FASTAQM KAMA AMART - Formed in December from nine units in Aleppo province. Its Facebook page shows a visit by SMC leader Salim Idriss to Aleppo's urban frontline.
HARAKAT AL-NOUR AL-ISLAMIYA - Aleppo-based Islamists identified in a July video as one of the rebel factions involved in ongoing fighting in the northern town of Khan al-Assal.
(Reporting by Dominic Evans; Editing by Alastair Macdonald)