Syria: who are the rebel groups fighting against Assad?

Islamic State militants and Syrian government forces are said to be encircling the key city of Aleppo, in what some believe could be a tipping point in the three-year civil war.

Moderate rebels seized parts of Aleppo, the largest city in Syria, in 2012 but are now surrounded by hostile forces. If they are forced out it would leave the moderates without a significant presence in any Syrian city and could be the "death blow" to their revolt against Syrian President Bashar al-Assad, which began in 2011, says the Wall Street Journal.

Meanwhile, bolstered by their rapid advance across Iraq, Islamic State fighters have taken several towns and villages in the Aleppo province, something that could threaten rebel supply lines into the city.

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Some analysts believe Assad has been avoiding confrontation with Islamic State, allowing it to weaken the moderate rebels, but this might change when it comes to the battle for Aleppo.

The brutal civil war, which has killed more than 170,000 people, is anything but simple. Fighting takes place not only between Assad's forces and the rebels, but between the hundreds of rebel groups themselves. Opposition groups have mushroomed across Syria, with allegiances and rivalries shifting every day. Here are just some of the key players.

The Free Syrian Army (FSA)

The FSA was formed in 2011 by a group of defecting Syrian Army officers with the intention of defending civilians from violence by the state. It is a coalition of rebel groups, some of which have received support or assistance from Saudi Arabia, the US and Qatar at different times. While it appears structured on paper, it is essentially an umbrella term for highly localised and fragmented fighting groups on the ground. In December 2012, donor nations brokered the creation of a Supreme Military Council (SMC), hoping it could unify the disparate ranks, but it has failed to do so. Last year, the FSA was said to make up around 31 per cent of the opposition, but the Washington Post says disheartened fighters are "leaving their brigades in large numbers", sometimes to more extreme rebel groups.

Islamic State

Islamic State, formerly known as Isis, has become the most well-known rebel group since taking control of a huge swath of territory across Iraq and Syria, which forms the heart of its self-proclaimed caliphate. It is considered to be one of the most extreme rebel groups with a strict interpretation of Islamic law. Disavowed by al-Qaeda, the radical Sunni group is known for public crucifixions and the beheading of its enemies. Many fighters from other groups are defecting to the Islamic State after seeing its financial and military gains. The group's acquisition of several towns in the northern province of Aleppo is seen as a significant expansion. Western officials say Islamic State foot soldiers number anywhere from 10,000 to 20,000.

Al-Nusra Front

The US has designated al-Nusra Front a terrorist organisation with affiliations to al-Qaeda. It announced its existence in January 2012 with a video posted online, claiming responsibility for a number of suicide bombings in Syria. It has played a significant role in the fight against Assad's forces, launching guerrilla attacks on rural government targets. But it has recently lost territory to Islamic State militants. Analysts say al-Nusra Front has essentially lost control over the east, with members either withdrawing from their military positions or announcing their allegiance to the Islamic State. In what appeared to be a bid to win their defecting members back, it recently announced plans to establish an "Islamic emirate", similar to a caliphate.

Islamic Front

Last November, seven leading Islamist rebel groups announced that they were forming the largest rebel alliance in the conflict yet, the Islamic Front. They said they would be an "independent political, military and social formation" to topple the Assad regime. In May this year, Middle East analyst [4]Charles Lister estimated that they were capable of deploying up to 60,000 fighters. The Islamic Front's leader, Ahmed Issa al-Sheikh, was previously head of the Syrian Islamic Liberation Front (SILF), a coalition of Islamist factions aligned to the FSA. They too have pledged to build an Islamic state in any post-Assad Syria. However, the US has reportedly reined in Qatar's support for the Islamic Front, opposing the group for its hard-line ideology. Despite the Islamic Front not having formal ties with the al-Nusra Front or the Islamic State, there is overlap between the groups, says the Daily Beast.

Syrian Revolutionaries Front

The Syrian Revolutionaries Front formed in December 2013 and launched a campaign against the Islamic State earlier this year. Its units are mostly moderate Islamists, but the alliance does not appear to have strong ideological leanings. Its backbone is the Syrian Martyrs Brigade, a once powerful group from the northern province of Idlib. It has expressed support for the FSA but was recently defeated in several areas by the al-Nusra Front.

Democratic Union Party (PYD)

Turkey has been accused of being too quick to support any rebel group opposed to Assad, including the most extreme, reports the Washington Post. It is now struggling to contain spill-over violence on its own territory. With the threat of border shootouts and Islamic State fighters trying to recruit Turks, it this week entered co-operative talks with the Democratic Union Party (PYD). The party is a Syrian offshoot of the terrorist Kurdistan Workers' Party (PKK), which has fought a long guerrilla war against Turkey. However, it also controls swaths of Syria after Assad withdrew the majority of his forces from the Syria's Kurdish areas in July 2012.