"There are fears over the lives of thousands of civilians who are stranded in the pocket," said Rami Abdel-Rahman, head of the Syrian Observatory for Human Rights.
On Saturday, the Syrian Democratic Forces (SDF), a Kurd-led rebel group supported by a US-led international coalition, started an offensive aimed at expelling Islamic State militants from their last bastion in the eastern province Deir al-Zour, near the Iraqi border.
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"The battle is continuing and fierce, with the aim of pressing remnants of the [Islamic State] elements to surrender," Abdel-Rahman told DPA. "There is no real progress on the ground because the area is full of mines [planted by Islamic State] and has many civilians."
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The SDF said Saturday that its forces had started the final battle to capture the village of Baghuz, the last pocket still under Islamic State control in eastern Syria, after the evacuation of more than 20,000 civilians from the area to safe, nearby camps.
Syria's Kurds have played a major role in fighting Islamic State in war-torn Syria, gaining much of the territory once held by the radical group in energy-rich eastern Syria.