U.S. officials are working to mitigate tensions between two U.S. allies in the nascent attempt to oust the Islamic State from its chief stronghold in Syria.

President Obama has authorized airstrikes against ISIS, while relying on Turkey and other local forces to provide the ground forces that have worked to recapture ISIS-held territory in Syria. That coalition requires longterm enemies to work towards a common goal of defeating ISIS. Turkey is particularly worried about U.S.-backed Syrian Kurds, known as the YPG, whom it regards as affiliates of the PKK, a Kurdish group in Turkey that has been warring with the NATO-allied government for years.