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U.S.-backed Syrian forces say their push to defeat the Islamic State group in its last, tiny bastion in Syria has been slowed by the presence of civilians and scores of prisoners held by the extremists.

An official with the Syrian Democratic Forces, who goes by his nom de guerre, Ciyager, says IS militants are still holding some 300 prisoners, both civilians and SDF fighters, adding that their fate is unknown.

Ciyager says there are no negotiations to get them released.

An SDF statement says hundreds of IS fighters and their families surrendered Thursday.

Some fighters have surrendered in recent weeks, but hard-core militants, including many foreigners, are still holed up in the shrinking space along the eastern banks of the Euphrates River.