Syrian government ground forces pushed into eastern Aleppo on Tuesday, a monitoring group said, alleging that government helicopters had also dropped poisonous gas on the besieged rebel enclave.
"We can say the regime took almost a third of Masakin Hanano, which is a key eastern Aleppo neighborhood," Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights, told dpa.
"This is a key and important advance for the Syrian army since the assault on eastern Aleppo started," Abdel Rahman said.
The push into the only major urban centre still in rebel hands came as the UN envoy to Syria warned that the government now appeared highly confident of a quick military victory in the five-year-old conflict.
Speaking in Berlin, Staffan de Mistura said he believed that President Bashar al-Assad would use the time before Donald Trump takes office as US president in January to start a destructive new offensive.
The government has stepped up shelling of eastern Aleppo since last week in an attempt to dislodge opposition fighters from the northern city, which was Syria's largest before the war.
At least 25 civilians have been killed in government shelling on the area in the last 24 hours, Abdel-Rahman said.
Capturing eastern Aleppo would be a major strategic victory for al-Assad's regime, which in recent months has also succeeded in effectively starving rebels out of strategic areas on the outskirts of the capital Damascus.
The Observatory said that regime helicopters dropped at least four barrel bombs containing poisonous chlorine gas on rebel-held eastern Aleppo on Tuesday, causing several cases of suffocation.
Local activists confirmed the report. Activists previously accused the government of carrying out chlorine gas attacks on the besieged enclave in September.
The activists also said a man, his wife and their four children were killed after suffering suffocation during a similar attack on the enclave's al-Sakhour neighbourhood on Sunday.
The Observatory confirmed that that attack took place but could not confirm that chlorine gas was used.
Damascus has denied use of the gas. In August, a joint probe by the UN and the Organisation for the Prohibition of Chemical Weapons found that Damascus had used chlorine gas as a weapon twice since 2014.
Syria disposed of its stockpile of banned chemical weapons under a 2013 UN Security Council resolution following a gas attack that killed hundreds in rebel-held suburbs of Damascus.
Holding stocks of chlorine is permitted as it has many civilian uses, but using any poisonous chemical as a weapon is banned under international law.
Also on Tuesday, the Observatory said that rebel groups were blocking some 100 families from leaving eastern Aleppo for a district controlled by Kurdish forces.
Government ally Russia has previously accused rebels of preventing civilians from using "humanitarian corridors" it says it has established to allow residents to leave the enclave.
However, the UN said security issues and fear of arrest by government forces were preventing people from leaving.
An estimated 250,000 to 300,000 people are under siege in eastern Aleppo, where residents report declining or disrupted supplies of basic necessities, including food, water, electricity and medicine.
Earlier, leaflets were dropped on the area bearing the name of the Syrian army and calling on residents not to leave their houses except in case of necessity and to stay away from places where armed groups were present.
© 2019 dpa GmbH