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.

"We're aware of Turkey's concerns regarding YPG," State Department spokesman Mark Toner told reporters. "We've talked at length about how we believe that while the PKK is a foreign terrorist organization, we don't necessarily draw that same conclusion about the YPG, that they're a very effective local fighting force in northern Syria."

Turkey has attacked Syrian Kurds in recent months, but those attacks might undermine the coalition offensive against Raqqa, the center of ISIS' self-declared caliphate, if they continue.

"We cannot extinguish the fire in our neighbor's house if our home is burning," said Rezan Hiddo, an official with the Syrian Defense Forces.

Turkey doesn't want the Syrian Kurds to achieve any military victories that would allow them to govern territory on the Turkish border with Syria. When the YPG liberated an ISIS-held town in Syria on the side of the Euphrates River nearest Turkey, the U.S. had to take action to keep the coalition together. Vice President Joe Biden warned the Syrian Kurds that they would lose U.S. support if they did not withdraw back across the Euphrates.

"As I said, we've asked that they live up to the commitments that they've made with regard to remaining east of the Euphrates," Toner reiterated on Monday.

U.S. officials are currently negotiating with the Turks to determine which forces ultimately will capture and then govern Raqqa as the broader Syrian civil war unfolds. "We always knew the SDF wasn't the solution for holding and governing Raqqa," Marine Corps Gen. Joe Dunford, chairman of the Joint Chiefs of Staff, said Monday after a meeting with Turkish military leadership. "What we are working on right now is to find the right mix of forces for the operation."