"After our conversation yesterday [Sunday] evening, as Mr. President [Donald Trump] stated, the retreat has begun," Erdogan told reporters at Ankara's Esenboga International Airport before his departure for a two-day visit to Serbia.

The U.S. on Sunday ruled out any assistance or involvement in Turkey's planned operation east of the Euphrates River in northern Syria.

In a statement, the White House said Turkey will soon carry out its "long-planned operation" into northern Syria.

"The United States Armed Forces will not support or be involved in the operation, and United States forces, having defeated the ISIS [Daesh] territorial ‘Caliphate,’ will no longer be in the immediate area," said the statement, using an alternate name for the terror group Daesh.

Since 2016, Turkey’s Euphrates Shield and Olive Branch operations in northwestern Syria have liberated the region from YPG/PKK and Daesh terrorists, making it possible for Syrians who fled the violence to return home.

Turkey has long decried the threat from terrorists east of the Euphrates in northern Syria, pledging military action to prevent the formation of a "terrorist corridor" there.

In its more than 30-year terror campaign against Turkey, the PKK -- listed as a terrorist organization by Turkey, the U.S. and the EU -- has been responsible for the deaths of some 40,000 people, including women, children and infants. YPG is the Syrian branch of the terrorist organization PKK.

More about: Turkey   Erdogan   United-States   Syria