UNICEF has called for a truce so that civilians who want to leave the city can do so "in safety and dignity"
More than 100 unaccompanied children are reportedly trapped in a building under heavy attack in eastern Aleppo, according to UNICEF.
Citing, "alarming reports from a doctor in the city" the regional director of the UN children's agency, Geert Cappelaere, said, "many children, possibly more than 100, unaccompanied or separated from their families, are trapped in a building, under heavy attack in east Aleppo."
Mr Cappelaere said it was "time for the world to stand up for the children of Aleppo and bring their living nightmare to an end."
He was speaking as pro-government forces continued their assault on Aleppo's few remaining rebel-held areas, trapping thousands of civilians.
President Bashar al Assad's forces, backed by Russia, have recaptured large swathes of the city in recent days and, according to state TV, it now holds 98% of neighbourhoods previously occupied by rebels.
The army said its troops and allied forces are in the "last moments before declaring victory", while the head of the government's Aleppo security committee warned rebels they "either have to surrender or die."
Cappelaere says that UNICEF is "deeply concerned" over the unverified reports of the "extrajudicial killings of civilians, including children" by Syrian government forces.
The UN human rights office said it had reports of Syria pro-government forces killing at least 82 civilians in the eastern part of Aleppo, including 11 women and 13 children, in four different neighbourhoods.
UNICEF called for a truce between the fighting parties and for civilians who wanted to leave the city to be allowed to do so "in safety and in dignity."