Turkish troops have begun an offensive in northern Syria, which could bring them into direct conflict with Kurdish-led forces allied to the United States.
The move was announced by Turkey’s President Recep Tayyip Erdogan.
Turkey wants to create a “safe zone” cleared of Kurdish militias which will also house some of Turkey’s 3.6 million Syrian refugees.
Donald Trump controversially withdrew US troops ahead of the action but says Turkey may face economic consequences.
The Kurds were key US allies in defeating the jihadist group Islamic State (IS) in Syria.
Mr Trump’s withdrawal of the few dozen US troops in the border area was seen as a “stab in the back” by the Kurdish-led Syrian Democratic Forces (SDF) and drew condemnation even from the president’s Republican allies.
He responded to the criticism by threatening to “obliterate” Turkey’s economy if it went “off limits” in the incursion, saying Turkey should “not do anything outside of what we would think is humane”.
The Kurds guard thousands of IS fighters and their relatives in prisons and camps in areas under their control and it is unclear whether they will continue to be safely detained.
Announcing the offensive, Mr Erdogan said on Twitter: “The Turkish Armed Forces, together with the Syrian National Army [rebel groups backed by Turkey], just launched #OperationPeaceSpring” against Kurdish militias and the Islamic State group in northern Syria.
“Our mission is to prevent the creation of a terror corridor across our southern border, and to bring peace to the area.
“We will preserve Syria’s territorial integrity and liberate local communities from terrorists.”