It has been warned Aleppo is experiencing a "descent into hell" and could witness one of the worst massacres since the Second World War as the Syrian government continues its brutal offensive on the city.

After four months of siege and a bombing campaign which has brought the rebel-held east to its knees, civilians are fleeing across the frontline in their thousands to escape President Bashar al-Assad’s attempts to crush the city’s rebels for good.

A sweeping advance by the Russian-backed Syrian army and allied militias has displaced thousands of people, the UN said on Tuesday, leaving residents unsure where to turn to for safety as the frontline fighting rapidly moves and rebels struggle to maintain control of key neighbourhoods.

The true figure once those who have fled to government areas is included is likely to be much higher.

On Wednesday, the UN Security Council will hold an emergency meeting in New York to discuss the humanitarian crisis. Ahead of that meeting, the French UN ambassador, Francois Delattre, warned: "France and its partners cannot remain silent in the face of what could be one of the biggest massacres of civilian population since World War Two".

Russia, meanwhile, has claimed that the regime air strikes and ground assault on east Aleppo have managed to “liberate” the area.

“Half of the territory in parts of eastern Aleppo occupied by militants in recent years has been completely freed,” Russian defence ministry spokesperson Maj Gen Igor Konashenkov told reporters.

“Most importantly, more than 80,000 Syrians have been liberated, tens of thousands children. Many received for the first time water, food, medical care from Russian humanitarian centres,” he stressed.

“Our Western counterparts are showing surprising blindness when it is time to assess the real situation in Aleppo.”

Activists on the ground, for their part, have condemned what they see as a continued silence from Western politicians in the face of what a World Food Programme spokesperson described as the city’s “slow descent into hell.” Doctor Abdul, working out of a makeshift facility, said that the decimated medical services – east Aleppo has approximately 30 doctors left – were struggling to cope.

The Bustan al-Basha neighbourhood in east Aleppo has seen heavy destruction in the assault (AFP/Getty) (AFP/Getty Images)

Another sent pictures to The Independent of what he said were the bodies of casualties from overnight still lying on the streets.

Wissam Zarqa, a teacher and rebel activist, also sent Whatsapp recordings to journalists on Tuesday saying that while the opposition has managed to accommodate hundreds of fleeing families in their rapidly shrinking territory, tripling the occupancy of some buildings, the fate of many of those who crossed into government-held areas or stayed put as the Syrian army retook their neighbourhoods is unknown.

He is worried about two people he knows well who have been arrested. "Hopefully they will be OK but I am not optimistic about that now,” he said.

Fear of arrest and interrogation by the army or Syria’s infamous mukhabarat, or secret police, has long been one of the major factors stopping the 250,000 people trapped inside east Aleppo from leaving, despite regime and Russia exhortations it is safe do so.

Dahiyet al-Assad - Aleppo Show all 16 1 /16 Dahiyet al-Assad - Aleppo Dahiyet al-Assad - Aleppo A girl who fled areas of conflict rides a vehicle in Dahiyet al-Assad, west Aleppo city, Syria REUTERS/Ammar Abdullah Dahiyet al-Assad - Aleppo Rebel fighters from the Jaish al-Fatah (or Army of Conquest) brigade have a tea in a building under construction Getty Dahiyet al-Assad - Aleppo A rebel fighters' armoured vehicle in Dahiyet al-Assad Reuters Dahiyet al-Assad - Aleppo Rebel fighters from the Jaish al-Fatah (or Army of Conquest) brigades sit on a tank Getty Dahiyet al-Assad - Aleppo Abandoned magazine of shells after rebel fighters took control of Dahiyet al-Assad Reuters Dahiyet al-Assad - Aleppo Rebel fighters ride a pick-up truck with civilians who fled areas of conflict in Dahiyet al-Assad, west Aleppo city, Syria Reuters Dahiyet al-Assad - Aleppo A rebel fighter gestures with a girl who fled areas of conflict while they ride a pick-up truck in Dahiyet al-Assad Reuters Dahiyet al-Assad - Aleppo Smoke rises near a damaged road in Dahiyet al-Assad, west Aleppo city, Syria Reuters Dahiyet al-Assad - Aleppo Syrians carry their belongings as they leave the southwestern frontline neighbourhood of Dahiyet al-,Assad Getty Dahiyet al-Assad - Aleppo A view shows a damaged minaret of a mosque after rebel fighters took control of Dahiyet al-Assad, Syria Reuters Dahiyet al-Assad - Aleppo Rebel groups have pledged to push from newly captured positions in the Dahiyet al-Assad district towards Hamdaniyeh. Rebels and allied jihadists launched a major offensive on October 28, 2016 to break through government lines and reach the 250,000 people living in the city's east Getty Dahiyet al-Assad - Aleppo Rebel groups have pledged to push from newly captured positions in the Dahiyet al-Assad district towards Hamdaniyeh Getty Dahiyet al-Assad - Aleppo Rebel fighters from the Jaish al-Fatah (or Army of Conquest) brigades hold a position at an entrance to Aleppo, in the southwestern frontline neighbourhood of Dahiyet al-Assad Getty Dahiyet al-Assad - Aleppo Smoke billows from the frontline district of Dahiyet al-Assad following an attack by rebels on Syrian regime forces in the northern city of Aleppo Getty Dahiyet al-Assad - Aleppo Syrians carry their belongings as they leave the southwestern frontline neighbourhood of Dahiyet al-Assad Getty Dahiyet al-Assad - Aleppo People who fled areas of conflict ride a pick-up truck in Dahiyet al-Assad, west Aleppo city, Syria Reuters

Death continued to come from the skies too, the White Helmets rescue service reported, in the form of renewed air strikes – including crude barrel bombs – dropped overnight and on Tuesday. At least 10 people were killed by the shelling, the UK-based Syrian Observatory for Human Rights said, with the death toll likely to rise as dozens of people are still wounded or missing.

Aleppo, once the country’s most populous city and its economic powerhouse, has been roughly divided into a government-held west and rebel-controlled east side since 2012.

Once known as the “Jewel of Syria”, it has been subject to some of the most brutal fighting in the five-year-long civil war.

Retaking the city for good will be a military and symbolic victory for Mr Assad, effectively removing the last so-called moderate rebels from Syria’s cities and relegating the opposition to the far north and south of the country.

Boy asks if he will die after alleged chlorine attack in Aleppo

Speaking from Brussels on Tuesday, the UN’s envoy to Syria, Staffan de Mistura, said he could not say how long rebels would be able to hold out against the massive new assault.

”This is a military acceleration and I can't tell you how long eastern Aleppo will last,” he told the European Parliament. “There is a constant increase of movement on the military side.”

Moscow and Damascus have stepped up the campaign in order to finish the battle for Aleppo before US President-elect Donald Trump takes office in January, a senior Syrian government official reconfirmed on Tuesday.