Up to 16,000 civilians have been displaced by the Syrian government's advance into besieged rebel-held areas of the city of Aleppo, the UN says.
Humanitarian chief Stephen O'Brien said thousands more were likely to flee if the fighting continued to spread and intensify in the coming days.
He expressed concern about their fate, calling the situation deeply alarming.
Troops and militiamen have retaken more than a third of the rebel-held eastern half of Aleppo since the weekend.
Overnight, at least 18 people were killed in government air strikes on the remaining rebel-held areas, including 12 in the Shaar district near the new northern frontline, according to a UK-based monitoring group.
Another 10 died in an air strike in the Bab al-Nairab area on Tuesday morning, the Syrian Observatory for Human Rights added.
An opposition activist network said a group of displaced civilians had been targeted and it put the death toll at 25.
It has been divided in roughly two for the past four years, with the government controlling the west and rebels the east.
But in the past year, Syrian troops have broken the deadlock with the help of Iranian-backed Shia Muslim militias and Russian air strikes.
In early September they reinstated a siege of the east, trapping the 275,000 people living there, and launched an all-out assault later that month.
Government forces stepped up their offensive two weeks ago and broke through the rebel lines on Saturday. By Monday, they had recaptured the entire northern half of the rebel enclave.
"It's something that can't be imagined," Ahmad Aziz, a resident of a rebel-held area said. "We are witnessing the worst days ever. We can't move and see each other because of crazy shelling."
Mr O'Brien said he was "extremely concerned about the fate of civilians as a result of the deeply alarming and chilling situation unfolding in Aleppo".