Turkish forces are stepping up air strikes and a ground offensive, as their incursion into Kurdish-held areas of northern Syria enters a second day.
Turkey’s military said it had seized designated targets. There are reports of heavy fighting in the central border region, and seven civilian deaths.
Tens of thousands of people are reported to be leaving their homes.
The assault on Kurdish-led forces, key US allies, follows US President Donald Trump’s decision to withdraw US troops.
Turkey says it wants to create a “safe zone” on the border for many of the Syrian refugees on its territory.
On Wednesday, US Secretary of State Mike Pompeo denied the US had given Turkey a “green light” for the offensive.
But Mr Trump told a news conference the Turks and Kurds had “been fighting each other for centuries”, and said that Kurdish fighters “didn’t help us in the Second World War, they didn’t help us with [the D-Day landings in] Normandy”.
“With all of that being said, we like the Kurds,” he added.
Republicans and Democrats alike have condemned the decision to pull back US troops.
What is happening on the ground?
Kurdish sources report a large ground offensive between the towns of Ras al-Ain and Tal-Abyad, in the central area of Syrian’s northern border with Turkey.
Turkish-backed Syrian rebels from the Free Syrian Army have also been involved in the fighting.
The area is sparsely populated and mainly inhabited by Arabs.