The abductions came as Russian and Syrian jets pounded rebel positions in second city Aleppo, an AFP correspondent and the Syrian Observatory for Human Right monitor said.
The Arab-Kurdish alliance known as the Syrian Democratic Forces (SDF) expelled most of the IS fighters from Manbij last week, but dozens continued to put up a tough resistance.
Today they withdrew from a district in northern Manbij heading for the IS-held town of Jarabalus along the border with Turkey, taking the captives with them.
"They used these civilians as human shields as they withdrew to Jarabulus, thus preventing us from targeting them," he said, adding that women and children were among those taken.
The Britain-based Observatory, which relies on sources on the ground to cover the conflict, also reported that IS had abducted around 2,000 civilians as they fled Manbij.
It said IS confiscated residents' cars, forced civilians into them and then headed for Jarabulus.
In January, IS abducted more than 400 civilians, including women and children, as it overran parts of Deir Ezzor province in eastern Syria. It later released around 270 of them.
IS has also used civilians as human shields, booby- trapped cars and carried out suicide bombings to slow advances by their opponents and avoid coming under attack.
Thousands of civilians were held captive by the group in Fallujah which Iraqi forces recaptured in June after a four- week offensive.
It was not immediately clear how many jihadists fled Manbij which the SDF captured on August 6.
But Darwish said that SDF managed to rescue 2,500 civilians who had been held captive by IS fighters there.
The US-backed forces combed Al-Sirb today for any remaining jihadists, he added.
With air support from the US-led coalition, the SDF began its assault on Manbij on May 31, surging into the town itself three weeks later.
But their offensive was slowed by a massive jihadist fightback, before a major push last week saw the SDF seize 90 percent of the town.
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