ISTANBUL — Ever since the first batch of S-400 missiles landed just outside Ankara in 2019, Turkey’s acquisition of the Russian-made air defense system has been a central friction point in its relations with the United States.

Washington has repeatedly called on Turkey to remove the S-400s from its territory, citing concerns that their radars pose a threat to NATO defense infrastructure. Leading Turkish officials, however, maintained the missiles were necessary to bolster domestic security. This stance initially led to the nation's suspension from NATO’s F-35 fighter jet program and later, in December 2020, saw the imposition of US sanctions on Turkey over the purchase.

Throughout the discord, a top security concern for Ankara has been Washington’s cooperation with Kurdish forces in Syria known as the People’s Protection Units (YPG), which Turkish officials view as an extension of the Kurdistan Workers Party (PKK), a militant insurgency that has been at war with the Turkish state since the 1980s. Since 2014, the United States has worked with YPG forces to eradicate the Islamic State from northeast Syria.

Now, nearly three weeks into US President Joe Biden’s tenure, a top Turkish figure has hinted at a possible opening to settle the long-standing S-400 issue by linking it with Ankara’s concerns over Kurdish forces in Syria. In an interview with the Hurriyet newspaper published Tuesday, Turkish Defense Minister Hulusi Akar signaled that Ankara might be willing to compromise on the Russian missiles if Washington reevaluates its support to the YPG.