Syria rebels

They said civilians were in great danger and they would support any initiative to ease their suffering.

The US and five Western powers also put out a joint statement calling for an immediate ceasefire to allow aid into rebel-held areas.

The Syrian government has ruled out any further ceasefires.

Its ally, Russia, has called all remaining rebels in eastern Aleppo "terrorists".

The coalition of militias fought 73 days to drive out IS and say they freed 2,000 civilians being used as human shields on the final day.

IS captured Manbij, close to the Turkish border, two years ago.

Roads to Syria's embattled second city, Aleppo, and to the IS capital, Raqqa, pass through the town.

"After the liberation of Manbij, IS members won't be able to freely travel to and from Europe anymore," said Syrian Kurdish leader Salih Muslim.

The video that emerged on social media showed a visibly frightened boy lying in the back of a truck alongside what appeared to be several rebel fighters.

The rebels claimed the boy was a prisoner from Liwa al-Quds -- a pro-regime militia force made up mostly of Palestinians in Syria -- and that he was captured from an unofficial refugee camp north of war-torn Aleppo.

The footage then shows one of the men taking a knife to the child's head. The boy appears to be quite young, perhaps around 10, though CNN cannot verify his age or identity.