Rebels put up fierce resistance on Friday in a key district of Syria's battered Aleppo, where a regime offensive has left bodies in the streets and sparked global outrage.
The government assault on the northern city has spurred a mass exodus of tens of thousands of residents from the opposition-held east and prompted fresh calls by Russia for aid corridors.
President Bashar al-Assad's forces captured northeast Aleppo this week and were focused on seizing Sheikh Saeed, a large district on the city's southeast edges.
But anti-government fighters fought back strongly there overnight, rolling back recent government gains, the Britain-based Syrian Observatory for Human Rights said.
"The regime and allied fighters... wanted to take this neighbourhood at any cost, because capturing it would allow them to target all remaining rebel-held districts," said Observatory head Rami Abdel Rahman.
"But rebels put up ferocious resistance, because they knew they would be trapped if Sheikh Saeed fell," he added.
Abdel Rahman said opposition forces now once again controlled at least 70 percent of the neighbourhood.
Sheikh Saeed borders the last remaining sections of Aleppo still in rebel hands -- a collection of densely populated residential neighbourhoods where thousands have sought refuge from advancing regime forces.
In preparation for street-by-street fighting in these districts, hundreds of fighters from Syria's elite Republican Guard and Fourth Division arrived in Aleppo on Friday, the Observatory said.
The Observatory said four civilians were killed in rebel rocket fire on government-held areas, bringing to 59 the civilian toll in the city's west.
More than 300 civilians, including dozens of children, have been killed in east Aleppo since the government offensive began on November 15, according to the Observatory.
Intermittent clashes on Friday rocked a block of residential buildings on Aleppo's eastern edges, where advancing regime forces have sought to secure the road towards the airport.
AFP's correspondent in east Aleppo said ferocious clashes could be heard in the Tariq al-Bab district, where regime forces were advancing on Thursday.
Civilians had already totally emptied the adjacent neighbourhood of Al-Shaar, where a few rebels manned positions in the streets.
The escalating violence has been met with international outrage, including a UN warning that east Aleppo could become "a giant graveyard".
Russia on Thursday proposed setting up four humanitarian corridors into east Aleppo to bring in aid and evacuated severely wounded people.
Moscow has announced several humanitarian pauses in Aleppo to allow civilians to flee, but until the recent escalation, only a handful did so.
East Aleppo's residents have been wary of previous such offers because of Russia's support for Assad, including launching a bombing campaign in support of his forces in September 2015.
Dozens of families trickled out on Friday, adding to the more than 50,000 people who have poured out of east Aleppo into territory controlled by government forces or local Kurdish authorities, the Observatory said.
Among those fleeing are nearly 20,000 children, according to estimates by the UN's children's agency.
"What is critical now is that we provide the immediate and sustained assistance that these children and their families desperately need," UNICEF spokesman Christophe Boulierac told reporters in Geneva.
"It's a race against time, as winter is here and conditions are basic."
On a one-day trip to Beirut on Thursday, German Foreign Minister Frank-Walter Steinmeier pledged 50 million euros ($53 million) to organisations serving the "suffering population" of Aleppo.
He said the political transition in the United States after Donald Trump was elected president meant world powers should not "aim for a major political solution", but focus on "defusing the conflict".
"What we must achieve is an agreement on breaks in the fighting, hopefully leading to a ceasefire so those who are suffering so horribly can finally have their basic needs met," Steinmeier said.
The loss of east Aleppo -- a rebel stronghold since 2012 -- would be the biggest blow to Syria's opposition in more than five years.
More than 200 civil society groups on Thursday appealed to the UN General Assembly to take action on Syria because "there is no sign that the Security Council deadlock will end anytime soon".
They urged "UN member states to step in and request an Emergency Special Session of the UN General Assembly" to end violence in Aleppo and across Syria.
The conflict erupted in March 2011 with protests calling for Assad's ouster, and has since evolved into a bloody and highly globalised war that has killed more than 300,000 people.