More than 50,000 people have fled rebel-held east Aleppo in the last four days as government forces advance, the Syrian Observatory for Human Rights said Wednesday.
The Britain-based monitor said more than 20,000 people had fled to western neighbourhoods of the city held by the government, with another 30,000 moving to areas held by Kurdish forces.
Syrian troops have seized at least a third of eastern Aleppo since renewing their bid to recapture all of the battered second city just over two weeks ago.
The former rebel stronghold has been under a government siege for more than four months, with international aid rations exhausted and other food stocks dwindling.
The government has said passages are open for civilians or surrendering rebels to cross into the west of the city, and accuses opposition forces of trying to prevent residents from leaving.
Many have chosen to go from the east to neighbourhoods held by Kurdish forces, which are officially aligned with neither the regime nor rebels, such as Sheikh Maqsud in the city's north.
Once Syria's economic powerhouse, Aleppo has been ravaged by the conflict that began with anti-government protests in March 2011.
The rebel-held east, which fell from government control in 2012, has been particularly savaged by the conflict, with widespread destruction caused by repeated regime attacks.
In September, the army announced a bid to retake the city, and it began a new phase of that operation on November 15, making swift progress.
The loss of east Aleppo would be potentially the worst blow for rebel forces since the conflict began.