National Defense Minister Hulusi Akar and U.S. Defense Secretary Jim Mattis discussed the Syrian crisis in a phone call late Monday, a Defense Ministry source told Anadolu Agency (AA).

Akar and Mattis exchanged views on the Turkey-U.S. defense relations, Syria and the fight against terrorism, the source, who spoke on condition of anonymity due to restrictions on talking to the media, said.

Turkey and the U.S. have major differences in Syria. While Ankara sees the People's Protection Units (YPG) as the offshoot of the PKK, a group recognized as a terrorist organization by the U.S., the EU and Turkey, Washington has partnered with the YPG under the pretext of the fight against Daesh, with truckloads of weaponry and military equipment supplied to the group. Washington denies the group's connection to the PKK, despite strong ideological and organizational links.

However, earlier this month, former United States Ambassador to Turkey James F. Jeffrey, who is known to be closer to Turkey's perspective on the PKK, was appointed as Washington's special representative for Syria.

In a comment to Daily Sabah in September 2017, Jeffrey had said Ankara was "right about the PYD [the Democratic Union Party], the PKK and the SDF [Syrian Democratic Forces] relationship and thus, Turkey has every right to be concerned and press Washington constantly for reassurances and details regarding U.S.-YPG ties."

Syria has been locked in a vicious civil war since early 2011, when the Bashar Assad regime cracked down on pro-democracy protests – which erupted as part of the Arab Spring uprisings – with unexpected ferocity.