Some of the thousands of children caught up in the devastating siege of Aleppo are so traumatised that they have stopped crying.

According to the United Nations, at least 2,700 children were among 8,000 evacuees allowed to leave the war-torn Syrian city in recent days.

But many more remain trapped in the remaining rebel-held areas.

In video footage, aired by Channel 4 News, one toddler, called Aya, sits on a stretcher in Aleppo’s last hospital, her face covered in dust and dried blood.

As chaos engulfs the room around her, the toddler doesn’t cry.

Her mother, Um Fatima, the only surviving adult of three families whose apartment block was obliterated by a Russian or Syrian bomb, says the family was sleeping when their home was hit.

The footage shows the unimaginable horrors suffered by the last remaining civilians waiting for safe passage to neighbouring towns in the rebel-held province of Idlib.

Mahmoud, a teenage neighbour of Um Fatima, cradles his one-month-old brother Ishmael Mohamed who was suffocated to death in the apartment collapse.

He says: “God will avenge us against this oppressor (Assad).”

Two other small children, a brother and sister, are seen wandering the blood-stained hospital corridors.

They are dust-covered and shell-shocked, looking for their mother. Neither is crying.

A nurse is heard asking adults: “Are these your children?”

Evacuation efforts of east Aleppo were halted on Friday following reports of a breakdown in the agreed ceasefire.

"Aleppo is now a synonym for hell," UN Secretary-General Ban Ki-moon said, urging a resumption of the evacuation.

Barack Obama accused Syrian President Bashar al-Assad, along with his Russian and Iranian allies, of committing “atrocities”.

“The world as we speak is united in horror at the savage assault by the Syrian regime and its Russian and Iranian allies on the city of Aleppo,” he said.

“It should be clear that although we may achieve tactical victories, over the long term, the Assad regime cannot slaughter its way to legitimacy.”

Speaking from the White House, the US President also called for a broader ceasefire to facilitate a political solution.

Aleppo before the Syrian Civil War Show all 12 1 /12 Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A man crosses a street in Aleppo, December 12, 2009 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A vendor sits inside an antique shop in al-Jdeideh neighbourhood, in the Old City of Aleppo, December 12, 2009 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A view shows part of Aleppo's historic citadel, overlooking Aleppo city, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A view shows part of Aleppo's historic citadel, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War Visitors walk inside Aleppo's Umayyad mosque, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War People walk inside the Khan al-Shounah market, in the Old City of Aleppo, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A man walks past shops in al-Jdeideh neighbourhood, in the Old City of Aleppo, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War People walk along an alley in al-Jdeideh neighbourhood, in the Old City of Aleppo, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War Visitors tour Aleppo's historic citadel, Syria December 11, 2009 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A general view shows the Old City of Aleppo as seen from Aleppo's historic citadel, Syria December 11, 2009 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War People walk near Aleppo's Bab al-Faraj Clock Tower, Syria October 6, 2010 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A man stands inside Aleppo's historic citadel, overlooking Aleppo city, Syria December 11, 2009 Reuters

“We all know what needs to happen, there needs to be an impartial, international observer force in Aleppo. They can help coordinate an orderly evacuation through safe quarters. There has to be full access for humanitarian aid,” he said.

“Beyond that there needs to be a broader ceasefire that can serve as the basis for a political rather than a military situation.”