As many as 100 children may be trapped inside a building which is under heavy attack in the Syrian city of Aleppo, Unicef said.
In a 13 December statement, Geert Cappelaere, Unicef's regional director for the Middle East and North Africa said: "According to alarming reports from a doctor in the city, many children, possibly more than 100, unaccompanied or separated from their families, are trapped in a building, under heavy attack in east Aleppo.
"We urge all parties to the conflict to allow the safe and immediate evacuation of all children. Unaccompanied and separated children have the right to be registered by a neutral humanitarian organisation and be reunified with their families – wherever they may be."
It comes after the United Nations human's rights agency (UNHCR) said it was receiving reports of women and children being killed 'on the spot' as Syrian forces retook areas of east Aleppo.
The office of the UNHCR said it had reliable evidence that in four areas 82 civilians were shot on sight.
"Unicef is deeply concerned by reports of extra judicial killings of civilians including children and reminds all parties of their responsibilities under international law," said Cappalaere.
The reported killings have been condemned as war crimes by Amnesty International.
"The reports that civilians - including children - are being massacred in cold blood in their homes by Syrian government forces are deeply shocking but not unexpected, given their conduct to date. Such extrajudicial executions would amount to war crimes," said Lynn Maalouf, Deputy Director for Research at the Beirut Regional Amnesty International office.
Former International Development Secretary Andrew Mitchell, who called an emergency debate on the crisis in the House of Commons, criticised parliament for its failure to act and alleviate the crisis.
"If we offer them nothing but empty words, nobody will believe them in the future," he said.
Humanitarian organisations need "unfettered access" to the area, he adds. "It is essential these teams are deployed. We need to get immediately food and medical supplies into eastern Aleppo."
Around 50,000 people are believed to be trapped in east Aleppo, a US official told the BBC. The Russian Centre for Reconciliation of the Opposing Sides said it has helped 7,796 civilians leave rebel-held areas in the last 24 hours.
In recent weeks the Syrian military backed by Russian air strikes has launched an offensive to seize control of areas of east Aleppo held by rebel groups.