The ceasefire in eastern Aleppo has collapsed with thousands of civilians, desperate to be evacuated, still trapped in the besieged part of the city.
Activists say some of the wounded are being buried alive because there is little medical help.
A deal brokered by Russia and Turkey yesterday was supposed to see civilians and rebel fighters evacuated by bus.
The buses waited for hours at the agreed position but eventually left empty because of fighting between government forces and the rebels.
United Nations senior advisor on Syria, Jan Egeland, says a ceasefire still might be possible, but he's not optimistic.
"What we hope is that this will lead to a safe exit of the civilian population in this remaining battlezone as it may also lead to a safe passage for ex-fighters out of the region.
"But nothing is secured, and too many times we've been disappointed earlier when we thought there was an evacuation and something went wrong."