Turkish ground troops have crossed into northern Syria as part of a major offensive to push out Kurdish militia, which Turkey regards as terrorists.
The Kurdish group targeted, known as the YPG, is active in the Afrin region, across from Turkey's southern border.
It says it has repelled Turkish troops in the area, and retaliated with rocket fire on Turkish border areas.
The militia forms a crucial part of a US-backed alliance battling Islamic State (IS) jihadists in Syria.
Turkish President Recep Tayyip Erdogan has vowed to crush the YPG "very quickly", but the US is urging Turkish "restraint" in order to avoid civilian casualties.
Turkey believes the group has links to the Kurdistan Workers Party (PKK), a banned militant group.
The Turkish government has for several months been threatening to clear Kurdish fighters from Afrin and another city, Manbij, some 100km (60 miles) away.