Aleppo battle: Syrian army presses advance against rebels
Syrian government troops pressed on with their offensive against rebel-held eastern parts of the city of Aleppo late into the evening.
The government has already recaptured more than 70% of the rebel-controlled areas in the east, the UK-based Syrian Observatory for Human Rights says.
More than 100,000 people are believed to be trapped in districts still in rebel hands.
Food supplies are exhausted and there are no functioning hospitals.
Reports say government troops are now targeting rebel-held parts of the Old City.
Earlier, they seized an entire district, al-Shaar, Syria's military said on Tuesday. This was confirmed by the Observatory.
The army also made gains in the districts of Sheikh Lufti and Marja, the monitoring group said.
Meanwhile, Syria's state TV reported that rebel armed groups were "collapsing" in three districts of the Old City.
It also announced that the area around Aleppo airport in the east had been "fully liberated".
The claims have not been independently verified.
In another development earlier on Tuesday, Russia accused the US of trying to buy time for rebels in Aleppo.
Foreign Minister Sergei Lavrov said the US had called off talks scheduled for Wednesday that were to focus on a possible rebel withdrawal.
"It looks like an attempt to buy time for the rebels, to have a breather, take a pause and replenish their reserves," he said.
US Secretary of State John Kerry rejected the suggestion that they were stalling talks.
"I'm not aware of any specific refusal," he said, on the sidelines of talks with Nato foreign ministers in Brussels.
Russia is a key ally of Syria's President Bashar al-Assad and has been carrying out air strikes against rebels since September 2015.
On Monday, Russia and China vetoed a draft resolution at the UN Security Council that called for a seven-day ceasefire in Aleppo.
Russia said the document infringed a council rule that allowed countries 24 hours to consider the final wording.
The US said Russia was trying to secure recent military gains by Syrian government troops in Aleppo.
It was the sixth time in five years that Russia had used its veto power to block a draft resolution on Syria.