Islamic State group fighters seized around 2,000 civilians to use as "human shields" as they fled their stronghold of Manbij in northern Syria, US-backed forces and a 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 tough resistance.
On Friday, they withdrew from a northern neighbourhood heading for the IS-held town of Jarabulus along the border with Turkey, taking the captives with them, as the Pentagon said the retreat showed the group was "on the ropes."
"While withdrawing from a district of Manbij, Daesh (IS) jihadists abducted around 2,000 civilians from Al-Sirb neighbourhood," said Sherfan Darwish, spokesman for the Manbij Military Council, a key component of the SDF.
"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 Syrian Observatory for Human Rights, which relies on sources inside Syria to cover the war, gave a similar report, saying IS forced around 2,000 civilians into cars it confiscated.
The jihadists, who have suffered a string of losses in Syria and Iraq, have often staged mass kidnappings when they come under pressure to relinquish territory they hold.
"Although fighting in Manbij continues, ISIL is clearly on the ropes. It has lost the centre of Manbij, it has lost control of Manbij," Pentagon deputy press secretary Gordon Trowbridge said, using another name for the group.