At least 13,000 civilians have fled the Islamic State group bastion of Manbij in northern Syria since the launch of a US-backed offensive there, a monitor said Monday.
The Kurds and Arabs fighting as the Syrian Democratic Forces alliance began their assault on Manbij in late May, encircling the town and entering its southwestern districts on June 23.
"At least 13,000 civilians have fled Manbij since the beginning of the SDF operation on May 31," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights monitoring group.
"The fleeing increased after the SDF besieged Manbij" on June 10, he said, adding that on Sunday "hundreds of people fled a southern district of the town where there have been fierce clashes in recent days".
Manbij had served as a vital stop along an IS supply route from Turkey, from the border town of Jarabulus to its bastion province of Raqa.
The SDF offensive on the town is backed by a US-led coalition that has been bombing IS in Iraq and Syria for nearly two years.
The UN's humanitarian office has not released its own estimates of how many people have fled Manbij, but said in late June that about 60,000 people were still in the town.
According to Abdel Rahman, residents are mostly fleeing from the southern SDF-controlled district into IS-free territory to the south.
The SDF transported one group north to the Kurdish stronghold of Kobane on the Syrian-Turkish border.
He said some civilians had died trying to escape, killed by IS snipers or explosives planted by the militants.
SDF fighters have been struggling to advance inside Manbij in recent days as IS has dispatched suicide attackers to defend the town.
"Daesh has used car bombs against a number of our positions," an SDF field commander told AFP on condition of anonymity.