U.S. Secretary of State Rex Tillerson called Foreign Minister Mevlüt Çavuşoğlu over Turkey's counter-terror operation in northern Syria's Afrin, diplomatic sources said Saturday.

The Turkish Armed Forces announced Saturday that Operation Olive Branch was launched in Afrin region to clear out terrorist groups such as the PKK and its affiliates KCK, PYD, YPG, and Daesh.

The official statement said that the operation started at 5:00 p.m. to neutralize terrorist groups and liberate friendly and brotherly people of the region from oppression and tyranny.

President Recep Tayyip Erdoğan previously said Saturday that Operation Olive Branch was "virtually" launched to improve Turkey's national security and prevent a "terror corridor" from being formed along its southern border.

The operation in Afrin – bordering Turkey's Hatay and Kilis provinces – was widely expected in the wake of Turkey's Operation Euphrates Shield in northern Syria, which cleared Daesh terrorists from Turkey's border between Aug. 24, 2016 and March 2017.