"I do not have the reasons. That has not been provided to me – why it was aborted l do not know. l hope it is only suspended," World Health Organization (WHO) Syria representative Elizabeth Hoff told reporters at the UN in Geneva by phone.
Though they do not know how many people remain in eastern Aleppo, Hoff said, "There is a very high number of people who should be evacuated."
Hoff said staff of the International Committee of the Red Cross and the WHO have been told to leave the area following the suspension.
Under the terms of an earlier agreement between the Assad regime and Syrian opposition groups, civilians trapped in eastern Aleppo would be allowed to go to the opposition-held city of Idlib.
Situated near the border with Turkey, Idlib is located some 65 kilometers (40 miles) from Aleppo.
In recent days, at least 7,500 civilians have left eastern Aleppo for Idlib, according to Syrian opposition sources.
Syria has been locked in a devastating civil war since early 2011, when the Bashar al-Assad regime cracked down on pro-democracy protests – which erupted as part of the Arab Spring uprisings – with unexpected ferocity.
Since then, more than a quarter of a million people have been killed and more than 10 million displaced across the war-battered country, according to the UN.
However, the Syrian Center for Policy Research, a Beirut-based NGO, has put the total death toll from the five-year conflict at more than 470,000.