President of Turkey, Recep Erdogan and President of United States of America, Donald Trump.

President Recep Tayyip Erdoğan held a phone call with his U.S. counterpart Donald Trump Monday to discuss bilateral and regional issues, including Ankara’s proposal on establishing a S-400 working group according to a report by Daily Sabbah

The two leaders exchanged views on recent developments in Syria and agreed to cooperate closely in the fight against terrorism, the Turkish presidency said in a statement.

The U.S. and Turkey are reportedly negotiating a plan to jointly patrol a 32-kilometer-deep safe zone in northern Syria along the Turkish-Syrian border.

In December last year, Trump announced that Washington would pull its troops out of Syria, saying that Daesh was defeated. The withdrawal decision was quickly interpreted as a sign of the end of U.S. support for the PKK-affiliated People’s Protection Units (YPG), which Turkey sees as a terrorist organization. Yet, in the face of mounting hints that the country will maintain its support for the terrorist organization and contradictory statements from officials on the withdrawal process, Erdoğan and Trump began discussing setting up a 32-kilometer safe zone to ease Turkey’s security concerns.

Ankara has been underscoring that the safe zone should be administrated by Turkey to ensure full elimination of security concerns and should be free from all terrorist elements, including the YPG.