Ankara has repeatedly told Washington to stop supporting the Kurdish Democratic Union Party and its armed wing the People's Protection Units (YPG), in northern Syria.
Turkey's government says the YPG are simply an extension of the country's Kurdish separatist militants, the Kurdistan Workers' Party (PKK), deemed a terrorist organization by both Ankara and Washington.
More than 40,000 people have been killed in over three decades of fighting between the Turkish state and the PKK. But Washington sees the Syrian-Kurdish militia as an effective fighting force against ISIL and therefore has been supporting them militarily, infuriating its ally Ankara.
