Turkey condemns 'escape' of Raqqa IS fighters
The Turkish foreign ministry described the information as an "extremely grave and eye-opening revelation", accusing the SDF of negotiating with IS to evacuate fighters.
The SDF is dominated by the Syrian Kurdish People's Protection Units (YPG), which Ankara views as a "terror" group linked to the outlawed Kurdistan Workers' Party (PKK) waging an insurgency inside Turkey.
Ankara has long lambasted Washington for working with and arming the YPG as the main US ally in the fight against IS.
The ministry said the purpose of the YPG was not to fight IS "but to create illegitimate faits-accomplis on the ground, to occupy territories and to alter their demographic structures," the ministry said.
Turkish President Recep Tayyip Erdogan has vowed he would not allow a "terror corridor" controlled by the YPG in Syria close to Turkey's border.
He has also warned that Turkey could yet mount a military operation against the YPG to oust the group from the northern Syrian town of Afrin.