Hundreds of elite Syrian troops moved into east Aleppo on Thursday ahead of a push into the most densely populated areas, after the UN warned the city risked becoming a "
Despite fierce global criticism, forces loyal to President Bashar al-Assad have pressed an assault to retake control of all of Aleppo, once Syria's commercial hub but now a divided city in ruins.
The assault -- backed by heavy artillery fire -- has spurred a mass exodus of tens of thousands of residents from rebel-held districts.
The relentless barrage has left Aleppo's streets strewn with the bodies of men, women and children, many lying next to the suitcases they had packed to escape.
The assault has seen Assad's forces make significant gains in the last week.
After overrunning the city's northeast, they were in control of 40 percent of the territory once held by opposition forces in Aleppo, according to the Syrian Observatory for Human Rights.
"The regime is tightening the noose on the remaining section of east Aleppo under rebel control," Observatory head Rami Abdel Rahman told AFP.
He said hundreds of fighters from the elite Republican Guard and Fourth Division arrived in Aleppo on Thursday "in preparation for street battles" in the densely populated southeast.
"They are moving in on the ground, but they are afraid of ambushes because of the density of both residents and fighters," he said.
The violence in Aleppo has sparked widespread outrage, but little concrete action from the international community.
Speaking to a special Security Council session on Wednesday, UN humanitarian chief Stephen O'Brien made an urgent appeal.
"For the sake of humanity we call on -- we plead -- with the parties and those with influence to do everything in their power to protect civilians and enable access to the besieged part of eastern Aleppo before it becomes one giant graveyard," he said.
Syrian warplanes have been pounding east Aleppo with air strikes for months -- often using crude munitions like barrel bombs -- but as the ground advance has gathered pace the army has instead turned to more precise artillery.
The effect has been no less devastating.
On Thursday, four children from a single family were killed in artillery fire by regime forces on the rebel-held Maadi neighbourhood of Aleppo, according to the Observatory.
And at least 26 civilians, including seven children, were killed in shelling of the rebel-held Jubb al-Qubbeh district on Wednesday, the Observatory said.
The latest attacks brought the civilian toll of the government's offensive to more than 300 civilians, including 42 children, since November 15.
Retaliatory rocket fire by the rebels on government-held areas has killed 48 civilians, according to the Britain-based Observatory, which has a wide network of sources on the ground.
Thousands of people have sought refuge in the remaining rebel-held neighbourhoods in southeastern Aleppo, arriving with overpacked suitcases or sometimes just the clothes on their backs.
Another 50,000 have poured out into territory controlled by government forces or local Kurdish authorities, according to the Observatory.
More than 300,000 people have been killed since Syria's conflict erupted in March 2011 with anti-government protests, before spiralling into a civil war.
The loss of east Aleppo -- a rebel stronghold since 2012 -- would be the biggest blow to Syria's opposition in more than five years.
The city had become a powerful symbol of Syria's uprising, producing some of the war's most iconic images -- including of Omran, the shell-shocked toddler in an ambulance.
At Wednesday's special UN session on Aleppo, British ambassador Matthew Rycroft said Assad ally Russia was hamstringing the Security Council.
Moscow launched a military campaign in support of Assad in September of last year and has since carried out of air strikes in Syria.
Rycroft accused Moscow, which in October vetoed a resolution to stop the bombing in Aleppo, of supporting "a deliberate act of starvation and a deliberate withholding of medical care."
Russia's envoy, Vitaly Churkin, brushed off criticism and said Syria was seeking to eliminate extremists such as the Al-Nusra Front, which has rebranded itself the Fateh al-Sham Front after severing ties to Al-Qaeda.
"We vehemently condemn any attempts to protect terrorists including any political action on a humanitarian pretext which, sadly alas, UN humanitarian works have been dragged into," Churkin said.
US ambassador Samantha Power urged the Security Council to pass a resolution that would mandate a 10-day military halt to allow humanitarian supplies to enter Aleppo.
But she feared a new Russian veto and acknowledged a brief halt "is barely even a Band-Aid and it is a sign in some ways of just how low our bar has become."