Syrian government forces and their allies have advanced in Aleppo overnight, seizing another neighbourhood from rebels, as they press an offensive to recapture all of the city, a monitor group and rebel sources said.
The UK-based Syrian Observatory for Human Rights said on Saturday that the capture of Tariq al-Bab means the government has now retaken the majority of the east of the city.
"We are told that around 50 percent of the rebel-held eastern Aleppo is now held by the government forces and its allies," Al Jazeera's Stefanie Dekker, reporting from Gaziantep on the Turkey-Syria border, said.
"This is the first time that we see this happening in four years. Aleppo has been in a stalemate between the government forces and rebels," she said.
"It is extremely concerning for those who still believe in this revolution," she added. "Activists say that Aleppo is the heartland of this revolution and if they lose this city, they would lose their civilisation, they would lose everything."
The rebels overran the east of Aleppo in mid-2012.
The latest advance also restored control of a road leading from government-controlled western neighbourhoods to Aleppo airport, which the regime also holds.
The government's capture of Tariq al-Bab came after ferocious clashes that sent civilians flooding out of the adjacent neighbourhood of al-Shaar.
More than 300 civilians have been killed in east Aleppo since the government resumed its offensive to remove the rebels on November 15, according to activists.
The Syrian Observatory says nearly 65 civilians have been killed in the same period by rebel fire on government-held west Aleppo, including nine on Friday.
The UN has warned that eastern Aleppo risks becoming a "giant graveyard" for the 250,000-plus civilians who were trapped there just last week. Tens of thousands have since fled.
No progress in Turkey talks
On the diplomatic front, tension between Syrian opposition groups and Russia escalated on Friday, with weeks of secret meetings in Turkey making little apparent progress on lifting the siege of eastern Aleppo.
Russia is a key backer of Syria's President Bashar al-Assad and has been bombing opposition-held areas since September last year.
Turkey backs the Syrian anti-government fighters and has been acting as a mediator in the meetings.
Syrian opposition officials told the Reuters news agency on Friday that Russia is not serious about the talks over a pause in the fighting.
Russia's proposals include expelling 200 fighters from Jabhat Fateh al-Sham, the armed group which changed its named from al-Nusra Front earlier this year when it said it cut ties with al-Qaeda.
Jabhat Fateh al-Sham is one of the largest and most powerful groups fighting the Syrian government, and is considered by both the United States and Russia to be a "terrorist" organisation.
Russia is also suggesting the creation of four "humanitarian corridors" to allow aid into besieged areas of eastern Aleppo.
Although Turkey has called for an immediate ceasefire, analysts contend that its priorities have shifted from Assad's removal to containing Kurdish groups seeking more territory on its border.
Source: Al Jazeera News and Agencies