A deal to evacuate rebel fighters and civilians from eastern Aleppo has stalled, with heavy shelling reported in the Syrian city.
A ceasefire was declared in Aleppo on Tuesday and buses brought in to ferry people out of the devastated enclave.
But fighting resumed on Wednesday. Syrian activists also say air strikes over rebel-held territory have resumed.
The breakdown of the deal, brokered by Russia and Turkey, is being attributed to demands from the government side.
It is said to be seeking the evacuation of injured fighters and civilians from nearby towns encircled by opposition forces.
The latest on the ground
Eastern Aleppo has been held by the rebels since 2012. But they have been squeezed into ever-smaller areas in recent months by a major government offensive, backed by Russian air power.
In recent days government troops have made sweeping gains. Russia's military said on Wednesday that rebels were confined to only 2.5sq km of the city.
Late on Tuesday, Russian UN Ambassador Vitaly Churkin told an emergency session of the UN Security Council that "military actions in eastern Aleppo are over".
Under the evacuation deal, civilians and rebels from eastern Aleppo were to be allowed to go to rebel-held areas in northern Syria.
Evacuations had been due to start at 05:00 (03:00 GMT), but did not go ahead. Fresh shelling was reported several hours later.