Syrian rebels are fighting for their survival in and around the northern city of Aleppo.

An assault by the Syrian army, backed by heavy Russian air power, severed the last rebel supply line from Turkey to Aleppo on Wednesday, in a devastating blow to the Syrian opposition.

On Thursday, the government captured several more villages in the surrounding countryside, leading to fears they could encircle the city.

Thousands of Syrians flee government offensive in Aleppo

The battle for Aleppo has fuelled opposition suspicions that the Syrian regime and its allies are more interested in securing a military victory over the rebels than negotiating a settlement.

Who is fighting whom?

Pro-government soldiers in the town of Tal Jabin, north of Aleppo, ahead of the assault to recapture Nubul and Zahraa (Getty Images/AFP)

Government forces, backed by Russian air support, are engaged in battle with the National Coalition of Syrian Revolutionary and Opposition Forces, the main opposition group.

Regime forces are also aided by Iranian advisers and Lebanese Hezbollah militia, according to the London-based Syrian Observatory for Human Rights.

In pictures: Russian air strikes in Syria Show all 19 1 /19 In pictures: Russian air strikes in Syria In pictures: Russian air strikes in Syria Syrian boys cry following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Aleppo Getty In pictures: Russian air strikes in Syria Russian defense ministry spokesman Major General Igor Konashenkov speaks to the media in Moscow, Russia. Konashenkov strongly warned the United States against striking Syrian government forces and issued a thinly-veiled threat to use Russian air defense assets to protect them AP In pictures: Russian air strikes in Syria Syrians wait to receive treatment at a hospital following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Alepp Getty In pictures: Russian air strikes in Syria Russian Deputy Defense Minister Anatoly Antonov speaks at a briefing in the Defense Ministry in Moscow, Russia. Antonov said the Russian air strikes in Syria have killed about 35,000 militants, including about 2,700 residents of Russia AP In pictures: Russian air strikes in Syria Jameel Mustafa Habboush, receives oxygen from civil defence volunteers, known as the white helmets, as they rescue him from under the rubble of a building following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Aleppo Getty In pictures: Russian air strikes in Syria Civil defence members rest amidst rubble in a site hit by what activists said were airstrikes carried out by the Russian air force in the town of Douma, eastern Ghouta in Damascus, Syria Reuters In pictures: Russian air strikes in Syria A girl carrying a baby inspects damage in a site hit by what activists said were airstrikes carried out by the Russian air force in the town of Douma, eastern Ghouta in Damascus, Syria Reuters In pictures: Russian air strikes in Syria Civilians and civil defence members look for survivors at a site damaged after Russian air strikes on the Syrian rebel-held city of Idlib, Syria Reuters In pictures: Russian air strikes in Syria Civilians and civil defence members carry an injured woman on a stretcher at a site damaged after Russian air strikes on the Syrian rebel-held city of Idlib, Syria Reuters In pictures: Russian air strikes in Syria Volunteers from Syria Civil Defence, also known as the White Helmets, help civilians after Russia carried out its first airstrikes in Syria In pictures: Russian air strikes in Syria The aftermath of Russian airstrike in Talbiseh, Syria In pictures: Russian air strikes in Syria Smoke billows from buildings in Talbiseh, in Homs province, western Syria, after airstrikes by Russian warplanes AP In pictures: Russian air strikes in Syria Russian Air Forces carry out an air strike in the ISIS controlled Al-Raqqah Governorate. Russia's KAB-500s bombs completely destroy the Liwa al-Haqq command unit In pictures: Russian air strikes in Syria Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria Â© TASS/ITAR-TASS Photo/Corbis In pictures: Russian air strikes in Syria Russia claimed it hit eight Isis targets, including a "terrorist HQ and co-ordination centre" that was completely destroyed In pictures: Russian air strikes in Syria A video grab taken from the footage made available on the Russian Defence Ministry's official website, purporting to show an airstrike in Syria In pictures: Russian air strikes in Syria A release from the Russian defence ministry purportedly showing targets in Syria being hit In pictures: Russian air strikes in Syria Russia launched air strikes in war-torn Syria, its first military engagement outside the former Soviet Union since the occupation of Afghanistan in 1979. Russian warplanes carried out strikes in three Syrian provinces along with regime aircraft as Putin seeks to steal US President Barack Obama's thunder by pushing a rival plan to defeat Isis militants in Syria In pictures: Russian air strikes in Syria Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria, a thousand kilometres away. The targets include ammunition factories, ammunition and fuel depots, command centres, and training camps Â© TASS/ITAR-TASS Photo/Corbis

Aleppo is divided between pro-Syrian forces and a range of rebel groups, from the Free Syrian Army to the radical al-Qaeda-affiliated Jabhat al-Nusra. These rebel groups are not all aligned in their fight against Assad, with the moderates that form the Syrian National Coalition distancing themselves from Islamist designated terror groups such as the al-Nusra front and Isis.

In addition, the Kurdish People's Protection Units (YPG) control an area of northern Aleppo.

Why is Aleppo significant?

Syrian pro-government troops hold positions in the Syrian town of Ain al-Hanash near l-Bab in Aleppo's eastern countryside on 26 January, 2016 (GEORGES OURFALIAN/AFP/Getty Images)

Rebels have maintained control of much of Aleppo since they took the city in 2012.

The loss of Aleppo to government forces would represent a potentially decisive blow to the rebellion against President Bashar al-Assad.

Such a loss could also significantly weaken Western-backed rebel groups fighting against Isis (which the regime says it is also fighting), further empowering the terror group in Syria.

What effect has the fighting had?

UN officials have said up to 20,000 Syrians are massing on the border (AFP)

As many as 20,000 refugees have fled the city as a result of the fighting, spending the night at the Bab al-Salam border crossing with Turkey.

The Turkish Prime Minister Ahmet Davutoglu said the camp would continue to feed and shelter the refugees, but did not say when or if they would be allowed to enter his country.

The battle could also exacerbate the humanitarian crisis, as the UN has found itself unable to deliver aid to towns besieged by government troops.

With Aleppo almost surrounded, there is a risk people living there could be cut off from aid deliveries entirely.

What will the long-term consequences be?

Battle for Aleppo threatens Switzerland peace talks

Tentative peace talks in Geneva were "temporarily halted" because of the government's major advance against rebel forces north of Aleppo.

The government's attack was condemned by France for "torpedoing" the Syrian peace talks and the US said it was "difficult" to see how the air strikes, which the State Department said were mostly on civilian targets, would help resolve the conflict.

US State Department spokesman John Kirby said the assault was mainly focused on opposition forces and urged Moscow to focus instead on its purported aim of fighting Isis.