On Monday rebel defences collapsed, leading to a broad army advance across more than half of the remaining insurgent pocket in Aleppo and a retreat of opposition fighters to a few districts on the west bank of the city's river.

Recapturing the entire rebel pocket of Aleppo will constitute the biggest battlefield victory yet for Syrian President Bashar al-Assad and his military coalition of Russia's air force, Iran and Shi'ite militias.

For rebels, it will mark a sobering loss and leave them without a significant presence in any of Syria's main cities. They still hold much of the countryside west of Aleppo and the province of Idlib, also in northwest Syria.

After days of intense bombardment of rebel-held areas, the rate of shelling and air strikes dropped considerably late on Monday and through the night, a Reuters reporter in the city said.

The military official said the rebels were fleeing "in a state of panic", but a Turkey-based official with the Jabha Shamiya insurgent group in Aleppo said late on Monday that they had established a new front line along the river.

Celebrations on the government side of the divided city lasted into Monday night, with the Reuters reporter there describing the bullets coming "like rainfall" as fighters shot into the air in triumph.

As the frontlines quickly shifted on Monday, however, thousands more people fled the fighting, carrying what possessions they could carry and sometimes pushing relatives in wheelchairs, before a heavy rainstorm began in the night.

The International Committee for the Red Cross issued a plea in an emailed statement early on Monday for all sides to spare civilian life.

The UN Secretary General Ban Ki-moon was alarmed by unverified reports of atrocities in the wake of the army's advance, his spokesman said late on Monday.

"As the battle reaches new peaks and the area is plunged into chaos thousands with no part in the violence have literally nowhere safe to run," the ICRC statement said.

Zakaria Malahifji, another Turkey-based official for the Fastaqim rebel group fighting in Aleppo said early on Tuesday that there had been no further international contacts over a proposal to spare the city by allowing fighters to withdraw.