Turkish warplanes have bombed parts of north-eastern Syria at the start of an offensive which could lead to conflict with Kurdish-led allies of the US.
Turkey's President Recep Tayyip Erdogan said the operation was to create a "safe zone" cleared of Kurdish militias which will also house Syrian refugees.
Several locations were hit and at least two civilians died, the Kurdish-led Syrian Democratic Forces (SDF) said.
It vowed to resist any Turkish advance across the border.
Amid growing humanitarian concerns, the SDF asked the US-led coalition against the jihadist group Islamic State (IS) to establish a no-fly zone in the area to stop "attacks on innocent people".
The Kurds - key US allies in defeating IS in Syria - 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.
The offensive was launched days after President Donald Trump withdrew US troops from the border area, a decision announced after a phone call with Mr Erdogan that was widely condemned.
In a statement, Mr Trump - who had threatened to "obliterate" Turkey's economy if it went "off limits" - said the US did not "endorse this attack" and had told Turkey that the operation was a "bad idea".
UK Foreign Secretary Dominic Raab expressed "serious concerns" about Turkey's offensive, saying it "risks destabilising the region, exacerbating humanitarian suffering, and undermining the progress made against" IS.