So far, the United States has been more than happy with the Turkish operations against the Islamic State in northern Syria. They have weakened the group tremendously by depriving it of resources, territory and fighters. But Washington's focus in the area has always been to drive the group from Raqqa , its self-proclaimed capital. To that end, the United States has also relied on the Syrian Democratic Forces coalition, dominated by the Kurdish militia group the People's Protection Units (YPG), which Turkey views as a threat to its national security. As Turkey's Operation Euphrates Shield progresses toward al-Bab, the spate of fighting between Turkish-backed rebels and the YPG is increasing. To the United States' displeasure, the fighting has distracted the YPG and could threaten to undermine the Raqqa effort altogether. Moreover, the United States is concerned that the Turkish-backed rebels are getting perilously close to loyalist lines.