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The UN envoy for Syria has said he is unsure how long the fighting in the rebel-held districts of Aleppo would last as it is still intensifying.
The Syrian army and its allies have made huge advances across the besieged eastern Aleppo in recent days with thousands forced to flee their homes.
"Clearly, I cannot deny - this is a military acceleration and I can't tell you how long eastern Aleppo will last," Staffan de Mistura told the European Parliament.
"There is a constant increase of movement on the military side."
Up to 16,000 people have been displaced in the Syrian city of Aleppo over the past few days following intense attacks in the rebel-held eastern part of the city, the United Nations has said.
Humanitarian chief Stephen O'Brien said the area has "no functioning hospitals left" and "official food stocks are practically finished".
Thousands more are expected to flee their homes in the coming days if the fighting persists, he added in a statement.
Warplanes pounded eastern Aleppo overnight killing at least 18 people, according to the Syrian Observatory for Human Rights - a UK-based war monitor.
The Syrian army and its allies made a huge advance across the northern part of besieged eastern Aleppo on Sunday night and Monday as rebels retreated to a more defensible front line after losing control of a key district.
Some 10,000 people are thought to have crossed over into government-held areas after Syrian forces made huge advances in east Aleppo.
Syrian state television showed jubilant scenes as people fleeing the frontlines were bussed to districts now under government control.
But many will be fearful of the future under Assad's regime where anything less than full support for the President could be a death sentence.
While many have escaped the rebel-held areas, thousands remain in these heavily bombed districts facing worsening circumstances as fuel and food become increasingly scarce.
Thousands of people have begun to flee districts of eastern Aleppo with government forces making significant advances into rebel-held territory.
Rebel defences appear to be rapidly failing with government-backed forces retaking a third of opposition-held Aleppo in recent days.
An estimated 250,000 people remain trapped in the besieged east of the city, with food supplies running perilously low.
And with the ongoing loyalist onslaught - believed to be the rebels' biggest defeat in Aleppo since 2012 - thousands of people are beginning to flee to government-held western districts of the city or deeper into rebel-controlled areas.
"The situation in besieged Aleppo (is) very, very bad, thousands of eastern residents are moving to the western side of the city," said Khaled Khatib, a photographer for the Syrian Civil Defense search-and-rescue group.
One Kurdish leader estimated that between 6,000 and 10,000 had fled.
Syrian government forces have seized a key district in eastern Aleppo as rebels lost further ground in recent days.
Troops loyal to Bashar al-Assad re-took the strategic eastern district of al-Sakhour in a wider advance that has driven rebels from a third of their territory in the besieged city, according to the Syrian Observatory for Human Rights.
It has also been reported that the rebels have lost control of the northern section of opposition-held eastern Aleppo.
Rami Abdulrahman, Director of the Observatory, said: "It is the biggest defeat for the opposition in Aleppo since 2012.
"The opposition has lost more than third of the area it controlled in Aleppo city during the big advance".
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Seven-year-old Bana al-Abed tweeted she "saw deaths and almost died" after heavy shelling in Aleppo overnight.