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BEIRUT (Reuters) - The battle for Aleppo, for months the focal point of Syria's multi-sided civil war, is approaching a bloody end as the army and its allies swept through rebel-held areas in recent days.
The historic city of Aleppo had for years been divided into a government-held west and rebel-held east, with the army leaning heavily on foreign militias and the rebels divided among an array of factions.
Finally taking Aleppo will give Syrian President Bashar al-Assad his biggest battlefield victory yet after nearly six years of conflict.
Here are the main combatants fighting in the city:
One of the main rebel groups fighting under the banner of the Free Syrian Army (FSA) in east Aleppo. Jabha Shamiya has received support from Turkey and other states.
There are numerous FSA groups, also including the Fastaqim faction, fighting in northern Syria as part of the nationalist opposition to Syrian President Bashar al-Assad. Many have received military aid from countries opposed to Assad, notably Turkey, Saudi Arabia and the United States.
The faction's Aleppo commander led a new military alliance that rebel groups formed this month in an attempt to better organise their defences against the government's attack. But he was seriously injured soon after.
A hardline Islamist group widely believed to have received backing from Turkey and some Gulf states.
It is an important component of Jaish al-Fatah, an alliance of disparate Islamist groups that made big gains against government forces last year.
Ahrar al-Sham is an influential rebel force, with a strong presence in northwestern Syria around Aleppo and Idlib.
A rebel group that counts itself part of the FSA umbrella, but has also recently participated in an operations room with the Islamist Jaish al-Fatah coalition.
It has received military support, channeled through Turkey, from states opposed to Assad, including U.S.-made TOW missiles. In July the group said it was conducting an internal investigation after the publication of a video that appeared to show its fighters beheading a child.
Formerly known as the Nusra Front, the powerful group changed its name to Fateh al-Sham in July and said it was breaking its formal allegiance to al-Qaeda.
It is estimated to have many thousands of fighters in Syria, including foreign jihadists, but the actual number in Aleppo is the subject of great controversy. Rebel groups have always said Fateh al-Sham has little to no significance inside east Aleppo.
Fateh al-Sham and other jihadist groups based outside the city attacked the southern Aleppo outskirts in October, taking part in a failed rebel offensive aimed at breaking the siege on the east in October.
The Syrian army is supported by Russian air strikes, local pro-government militias and mostly Shi'ite foreign militias.
Throughout the war, its air force has given it a big advantage over opponents, and the Russian intervention in Syria last year turned the tide in Assad's favour.
Military experts think the army numbered around 300,000 personnel pre-war, but after almost six years of conflict, desertions and defections, its current size is not known.
The army has launched several major campaigns to encourage recruitment, and formed a new corps of volunteers in November to fight alongside its regular soldiers.
Pro-Damascus sources say this elite unit of the Syrian army has played a leading role in the ground assaults in rebel-held eastern Aleppo. It is led by Suheil al-Hassan, an army officer who has risen to unusual prominence in the Syrian military.
The Lebanese Shi'ite movement gives allegiance to Iran's Supreme Leader Ayatollah Ali Khamenei. Its armed forces have long experience of military action after fighting numerous wars against Israel.
Hezbollah's entry into the Syrian war in 2013 helped stem rebel advances and it has played a significant role in the fighting around Aleppo. More than 1,500 of its fighters have died in Syria since the start of the war.
Iranian-sponsored Shi'ite militias have come to Syria from countries including Iraq, Afghanistan and Pakistan, to fight on the government's side.
Their manpower has played a critical role in the Aleppo battle and has helped fill the gap in the Syrian army's capacities.
Harakat al-Nujaba, an Iraqi Shi'ite militia fighting in Syria, sent more than 1,000 extra fighters to southern parts of Aleppo in September to reinforce its positions, the group's spokesman said.
A non-Shi'ite militia, the Liwa al-Quds, is made up of Palestinian refugees living in Syria. It took part in some of the army's recent advances in Aleppo.
The Kurdish YPG militia is at the heart of a U.S.-led campaign against Islamic State and it controls swathes of the north, where Kurdish groups associated with the militia have set up local government councils since the start of the war.
In and around Aleppo, the YPG has clashed with nationalist Syrian Arab rebels, which have accused it of collaborating with the government. The YPG has denied this accusation.
The YPG has mostly avoided conflict with the Syrian government, despite tensions flaring at some points. It controls the Sheikh Maqsoud district in northeast Aleppo.
Writing by Ellen Francis; Editing by Angus McDowall and Peter Millership