The Kurdish-led Syrian Democratic Forces (SDF) currently hold thousands of suspected IS prisoners.
Areas in north-eastern Syria under SDF control have come under heavy bombardment from Turkey since it launched its offensive on Wednesday.
More than 50 civilians have reportedly died on both sides of the border.
Turkey accuses the Kurds of being terrorists and has said it wants to drive them away from a "safe zone" reaching some 30km into Syria.
It also plans to resettle more than three million Syrian refugees in the zone who are currently in Turkey.
According to the United Nations, more than 100,000 people have already been displaced from areas under Kurdish control. The Kurdish administration in north-eastern Syria puts the figure much higher, at more than 191,000.
President Donald Trump's decision to withdraw US troops from the area effectively triggered the Turkish incursion against the SDF - the main Western allies in the fight against IS.