Former U.S. Defense Secretary Chuck Hagel said on Wednesday that former President Barack Obama's Syrian policy was "a fundamental mistake," adding that the issue was one of the differences he had with the Obama administration when he left the Pentagon.

"I was being hammered on by NATO allies, our Middle East allies. [They were asking] ‘What is your policy? What are you going to do? What are you trying to do?' We just didn't have one," he said in an interview with CNN.

Ankara had been criticizing the Obama administration due to its inconsistent and ambiguous policies on Syria, which had turned into a serious concern for Turkey as well as other allies at that time.

The close U.S. partnership with the People's Protection Units (YPG) initiated by the former president's administration had also been a sticking point in Washington-Ankara relations. Turkey sees the YPG as an extension of the PKK that has claimed the lives of more than 40,000 people in its 30-year terror campaign against Turkey. The U.S., however, while listing the PKK as a terrorist group, opted to continue its steadfast militarily support for the terrorist organization under the pretext of fighting Daesh despite warnings that the YPG does not represent Syrian Kurds.

U.S. Secretary of State Mike Pompeo also delivered a scathing rebuke of the Obama administration's Middle East policies yesterday as he denounced the former president for "misguided" and "wishful" thinking that diminished America's role in the region and harmed its longtime friends.

Hagel also criticized U.S. National Security Adviser John Bolton's attitude on Turkey, saying that the protection of the YPG must be assured as a precondition for the U.S. withdrawal.

"Bolton was saying what Turkey must do in order for us to leave. America says what a country must do before we leave somewhere. You ought to be very careful. That's certainly smacks of arrogance and other dimensions of a foreign policy that you don't want to have," he said.

Bolton said Sunday that the U.S. military withdrawal from northeastern Syria will be subject to defeating Daesh remnants, and Turkey assuring the safety of fighters allied with the U.S., i.e. the Syrian Democratic Forces (SDF), which is dominated by the YPG. His comments received a serious backlash from President Recep Tayyip Erdoğan, who reiterated that the YPG is a national security threat for the Turkish state as it is an extension of the PKK.