The United States has to be honest with Turkey and retrieve all the weapons supplied to the PKK's Syrian affiliate the People's Protection Units (YPG), which is the armed wing of the Democratic Union Party (PYD), after the operation to retake Daesh-held Raqqa is complete, a high-level United States military official in Ankara told Daily Sabah. Speaking to the paper on the condition of anonymity, the high level U.S. army official said, "There is a list of weapons provided to the YPG," and they "should be retrieved at the end of the Raqqa operation" as part of U.S. Secretary of Defense James Mattis' recent pledge to Ankara to do so.

"Washington really wants good relations with Turkey. The Trump administration has to be honest with Turkey for that," the army official said. Mattis recently sent a letter to Defense Minister Fikri Işık, promising that the United States would retrieve the weapons it sent to the YPG for the Raqqa battle after Daesh is defeated. The letter reportedly included information about the heavy weapons and equipment sent to the YPG. Mattis also pledged that the U.S. would share a list of materials given to the YPG each month with Turkey. The U.S. army official argued that the Trump administration had to work with the YPG because it could not see any other alternatives.

"In his election campaign Mr. Trump promised to finish off ISIS [Daesh] as the first thing to do during his presidency," the official said, explaining the reason behind the controversial cooperation with the YPG.

Washington's insistence on backing the YPG, which is considered as a terrorist group by Turkey for its ties to the PKK, has strained bilateral relations. The U.S. previously dropped 50 tons of weapons and ammunition to the YPG for the first time in October 2015. That airdrop was followed by heavy arms and weapons supply to the group in recent months for the Raqqa offensive.

The Turkish government expressed anger and frustration with the Trump administration for siding with a terrorist group rather than a crucial NATO ally. "At one side we will be together in NATO but on the other side you will act together with terror organizations. Then NATO needs to be reassessed. All of these moves are against NATO," President Recep Tayyip Erdoğan said on June 25.

Despite the Trump administration's pledge to retrieve all the weapons from the YPG once the operation is concluded, experts are expressing their doubts with regards to the possibility of it happening. Nurşin Ateşoğlu Güney, head of the international relations department at Yıldız Technical University, said there is a big question mark about how easy it would be to track all the YPG weapons or whose hands they would end up in.

"I think [Mattis' letter] is a goodwill gesture. It is an instrument to persuade Turkey. However, the question is how sure the U.S. can be on whether these weapons will be passed on to others' hands," Güney said, adding that weapons seized by the PKK in southeast Turkey are of western origin.

The Turkish government fears that the weapons supplied to the YPG will be directed at the Turkish military after they are passed on to the hands of PKK terrorists.

Haldun Yalçınkaya, an associate professor at the TOBB University of Economics and Technology who specializes in security operations and international relations, told Daily Sabah that the weapons obtained by the PKK are a primary concern for the Turkish army.

"These weapons threaten the security of the Turkish military. The PKK's capacity to use rocket launchers and missiles is dangerous," Yalçınkaya said.

Interior Minister Süleyman Soylu had previously said that the weapons were being placed in the hands of terrorist groups by their "allies" and they were being used against Turkish security forces. "Those who act like our allies and friends supply weapons to the PYD. You would be baffled if you saw the weapons seized in the caves [used by the PKK]. The weapons get in the hands of the PKK and are being used against us," Minister Soylu said in April.

Güney added that the U.S.-YPG cooperation is what troubles bilateral relations. "The U.S.'s efforts to convince Turkey will always lead to doubts so long as there is not an essential solution," Güney contended.

The YPG does not seem quite as enthusiastic about giving back the weapons. "We will not give up our weapons," a sniper in the YPG ranks taking part in the Raqqa offensive was quoted as saying in a Reuters article published in late June. Another YPG militant, who was named as Maryam Mohamed in the same article, said, "Erdoğan is our biggest enemy, we cannot hand over our weapons."

As the PKK-affiliated YPG looks very unwilling to hand over heavy weapons, ammunitions and other materials supplied by the U.S. once the Raqqa operation is over, Washington is left with a compelling task to fulfill its promise to Turkey and convince the terrorist group.