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.
Aleppo was Syria’s largest city and its commercial and industrial hub before the uprising against President Bashar al-Assad began in 2011.
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.
“The situation is very bad,” a medic who gave his name as Abu al-Abbas told Reuters news agency on Monday. “There’s intense fear of collective annihilation.”
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”.
“Intensified ground fighting and indiscriminate aerial bombardment over the past few days in eastern Aleppo city has reportedly killed and injured scores of civilians,” he added. “There are no functioning hospitals left, and official food stocks are practically finished.”
At the same time, Mr O’Brien noted, indiscriminate rebel shelling of civilian areas of government-controlled western Aleppo had killed civilians and displaced more than 20,000 in recent weeks.
Mr O’Brien said initial reports indicated that up to 16,000 people had fled their homes in eastern Aleppo in the past few days for government- and Kurdish-controlled districts, or other rebel-held areas.
The Syrian Arab Red Crescent and local non-governmental organisations were helping the displaced, and the UN was ready to deliver aid to rebel-held Aleppo, he added.
A spokeswoman for the UN high commissioner for human rights separately expressed concern that displaced people perceived to have links or connections to rebel groups might be detained by government or Kurdish forces.
Peaceful activists and relatives of rebels have been punished in the past for their provision of support to what the government considers terrorists, she added.
The parties to the conflict in Syria, Mr O’Brien said, had “shown time and again” that they were “willing to take any action to secure military advantage even if it means killing, maiming or starving civilians into submission in the process”.
Now at last, he pleaded, it was time to “restore basic humanity in Syria”.