The victims in the suicide car bombing Saturday at a rebel-held transit point where civilians were being evacuated included 68 children, and scores of others were injured.
"Someone pretending even to distribute aid and attracting the children produced that horrific explosion", UN Syria envoy Staffan de Mistura told reporters in Geneva.
The head of the UN's humanitarian taskforce for Syria, Jan Egeland, said at the same news conference that the assailants "disguised as a charity".
The blast, which tore through buses carrying civilians, also forced a pause in evacuations from the government- controlled towns of Fuaa and Kafraya.
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But the evacuations have resumed armed rebels were guarding civilians at the transit point on Thursday, according to AFP photographers at the scene.
Cars were being kept away from the site except for a Red Crescent vehicle that was allowed to distribute aid.
The evacuations are part of a deal between Syria's ally Iran and Qatar, a longtime supporter of rebel groups battling President Bashar al-Assad's forces.