In their attack Wednesday, government forces stepped up the use of airstrikes, including in Aleppo's Old City, according to a rebel official. The Observatory and government-allied media reported that troops were also battling opposition fighters on the edge of western Aleppo, which is controlled by the government.
Syrian forces have completely liberated an area in eastern Aleppo that is home to more than 90,000 civilians and has some 310,000 dwellings, Russia's center for reconciliation on warring parties said on Tuesday, according to a Russian news agency report.
Syrian opposition group National Coalition head Anas al-Abdeh demanded the United Nations "take immediate, definitive steps to protect civilians in Aleppo and stop the barbaric offensive taken against them" on Wednesday.
If the regime takes all of Aleppo - the last urban rebel stronghold in Syria - it would mark a turning point in the war, putting Assad's government back in control of all four major cities, making a political opposition less likely.
The group said a government artillery attack on a housing area on Wednesday morning, where those displaced by the fighting had sought refuge, killed at least 21 people.
The Syrian Civil Defense search-and-rescue group put the toll at 45 dead.
The Syrian government refers to all the rebels as terrorists. The United Nations and other global organizations choose not to offer humanitarian aid to Aleppo residents although there are no obstacles preventing aid deliveries, Head of the Main Operations Department at Russia's General Staff Lieutenant-General Sergei Rudskoi said on Wednesday.
A local activist told Amnesty International that families living in the area were unable to escape when government forces advanced and that many fear retaliation. The government has seized much of the northern half of the enclave in the past few days.
"Praise God, there are no warplanes", Ibrahim Abu al-Laith, a civil defence official in opposition-held Aleppo, said on Thursday.
Thousands of people have fled deeper into the rebel-held sector in recent days, as the government and its allies press on with a ferocious offensive aimed at taking the whole city.
He said there is no properly functioning hospital in eastern Aleppo, which has been under siege for almost 150 days and that most of the almost quarter million people trapped there don't have the means to survive much longer.
More than 50 000 Syrians have joined a growing exodus of terrified civilians from east Aleppo, as the UN Security Council was set for emergency talks on fighting in the city. They are now in control of more than 20% of eastern Aleppo.
The Syrian Observatory for Human Rights said on Wednesday that the Syrian government had detained hundreds of people forced to flee rebel-held areas of east Aleppo by the attack.