Elite Syrian troops have moved into east Aleppo ahead of a push into the most densely populated areas, as ally Russia called for corridors to bring in aid and evacuate the wounded.
Despite global criticism including the UN warning Aleppo risked becoming a "giant graveyard", government forces have pressed an assault to retake control of the divided city.
The offensive - backed by heavy artillery - has spurred an exodus of tens of thousands of residents from the rebel-held east.
It 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.
Artillery fire continued on Thursday but subsided as heavy rainfall hit the city.
The assault has seen President Bashar al-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 UK-based war monitor.
"The regime is tightening the noose on the remaining section of east Aleppo under rebel control," Syrian Observatory head Rami Abdel Rahman told the AFP news agency.
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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 residents and fighters," he said.
The head of the International Committee of the Red Cross in Syria appealed to all sides fighting in Aleppo to protect the civilian population.
"The people who're fleeing take a lot of risks. There is shelling, explosions, and sniper fire. People have left behind virtually everything," ICRC's Marianne Gasser, in Aleppo, said.
The violence in Aleppo has sparked widespread outrage at the Syrian government, but also at its steadfast supporter, Moscow.
On Thursday, Russia proposed setting up four humanitarian corridors into east Aleppo to bring in aid and evacuate severely wounded people.
However, Russian Foreign Minister Sergei Lavrov pledged military operations would continue until the city is "cleared of terrorists".
Russia announced "they want to sit down in Aleppo with our people there to discuss how we can use the four [humanitarian] corridors to evacuate people out", Jan Egeland, head of the UN-backed humanitarian taskforce for Syria, told reporters in Geneva.
He said Russia has pledged to respect the corridors, and that "we [the UN] now feel confident that the armed opposition groups will do the same".
Moscow has announced several humanitarian pauses in Aleppo to allow civilians to flee, but until the recent military escalation, only a handful did so.
Since Saturday, more than 50,000 people have poured out of east Aleppo into territory controlled by government forces or local Kurdish authorities, according to the Syrian Observatory.
Thousands more have sought refuge in the remaining rebel-held neighbourhoods in southeastern Aleppo, arriving with overpacked suitcases or sometimes just the clothes on their backs.
The loss of east Aleppo - a rebel stronghold since 2012 - would be the biggest blow to Syria's opposition in more than five years.
Syrian aircraft have been pounding east Aleppo with air strikes for months - often using crude munitions such as barrel bombs - but as the ground advance has gathered pace the army has instead turned to more precise artillery.
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 Syrian Observatory.
The government offensive has left 42 children dead, among a total of more than 830 people killed since November 15.
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Retaliatory rocket fire by the rebels on government-held areas has killed 48 civilians, according to the Syrian Observatory, which has a wide network of sources on the ground.
Meanwhile, rebels in Aleppo agreed to form a new military alliance to better organise the defence of parts of the city they control from the assault by the government and its allies, officials in two of the rebel groups said on Thursday.
Rivalry among rebel groups has been seen as one of their major flaws throughout the nearly six-year-old war.
The two officials, speaking from Turkey, said the new alliance would be called the "Aleppo Army" and led by the commander of the Jabha Shamiya rebel faction, one of the major groups fighting in northern Syria under the Free Syrian Army banner.
An official with a second rebel group confirmed that the Jabha Shamiya's Abu Abdelrahman Nour had been selected as the leader. In an interview with Reuters news agency last week, Nour urged greater support from foreign states that back the opposition.
The Jabha Shamiya group, known in English as the Levant Front, has received support from Turkey and other states that want Assad removed from power.
Source: Al Jazeera News and Agencies