Washington's support for the Kurdish People's Protection Units (YPG) is the biggest problem for Turkey, Defense Minister Hulusi Akar has said.
According to the Daily Sabah, Akar said the US' support for the YPG harms Turkey's relations with Washington more than Ankara's purchase of the Russian S-400 missile.
Turkey believes the YPG is the Syrian arm of the outlawed Kurdistan Workers' Party (PKK) which is designated as a terrorist group by Ankara, the US, and the EU.
"The US sees S-400 as threatening for our relations. But, the biggest problem is its support for YPG. The PKK terrorist group is getting at Turkish people and unity while the YPG is no different from the PKK. Washington sends trucks and planes loaded with arms to the YPG and cooperates with them. Stubbornly and insistently they say YPG and not PKK," Akar said.
Last year, the US argued that the S-400 missile defence system could be used by Russia to covertly obtain classified details on the Lockheed Martin F-35 jets if Turkey were to use the system it had purchased.
Ankara has, however, consistently maintained that the system would not be integrated into NATO systems and would therefore not pose a threat to the alliance.
Washington has said Turkey should end the presence of Russian personnel who are providing training and assembling the missile defence system. Last week, Turkey said it would do so.
Late last month, Turkey's Foreign Minister Mevlut Cavusoglu said he had told US Secretary of State Antony Blinken that Ankara's purchase of the Russian missile defence system was "a done deal".
"On the S-400s, we reminded them once again why Turkey had to buy them and repeated that Turkey had bought them and this is a done deal," Cavusoglu told reporters in Brussels.