Defense Secretary Ash Carter, saying the United States understands Turkey's long-standing historical "objections" to a Kurdish militia known as the YPG, called on the two sides to stop shooting at one another and retreat to agreed-upon geographical boundaries designed to keep the two sides apart.
"We have called upon Turkey to stay focused on the fight against ISIL and not to engage Syrian [Democratic] Forces," Carter said Monday at a Pentagon news conference with his Indian counterpart, Manohar Parrikar.
Carter said the U.S. has agreed to support both sides as long as Turkey confines itself to securing its border area and doesn't move farther south.
The U.S. also told some Kurdish YPG elements of the overall Syrian Democratic Forces to stay east of the Euphrates River to prepare for an eventual move south to confront the Islamic State forces in Raqqa.
Turkey considers the YPG a terrorist groups linked to attacks in Turkey.
"We do understand that they have historical differences with one another, but American interests are quite clear," Carter said. "We, like they, want to combat ISIL."
Carter said there have been a number of high level contacts with Turkey over the past few days as Turkish forces have bombed and shelled some of the Syrian fighters the U.S. is backing, and Kurdish troops have fired rockets at Turkish forces.
U.S. Joint Chiefs Chairman Gen. Joseph Dunford spoke with his Turkish counterpart by phone, and Carter said he'll be meeting with Turkey's defense minister next week in Europe.
The message from the U.S. is that it intends to work with both sides to fight the Islamic State, and it expects both of its allies to avoid coming into conflict with each other.
"We've called on both sides to not fight with one another, to continue to focus the fight on ISIL," Carter said. "That's the basis of our cooperation with both of them, and specifically not to engage one another, and to retain those geographic commitments that they have made."
Carter said the Pentagon is trying to clarify to Turkey where the YPG elements of the Syrian Democratic Forces are, in order to reassure Ankara that the Kurds are not establishing an autonomous Kurdish zone along Turkey's southern border.