Turkish ground forces have pushed into northern Syria's Afrin province as Ankara steps up artillery attacks on a US-backed Kurdish militia it aims to sweep from its border.
NATO's second-largest army entered northern Syria shortly after 11am local time, broadcaster HaberTurk cited Prime Minister Binali Yildirim as saying.
The Syrian-Kurdish YPG militia, supported by the United States but seen as a terrorist organisation by Turkey, said it had repulsed the Turks and their rebel allies after fierce clashes.
The fighting marks the second day of Turkey's new front in the nearly seven-year-old Syrian civil war.
Under what the Turkish government has called Operation Olive Branch, Turkish air strikes on Saturday pounded positions of the Syrian-Kurdish YPG militia in the northern Afrin province.
The military said it had hit 153 targets so far, including shelters and hideouts used by Kurdish militants. The YPG has said Turkey's strikes killed six civilians and three of its fighters, and wounded 13 civilians.
The YPG has also accused Turkey of striking civilian districts and a camp for the displaced in Afrin.
A Reuters reporter in the northern Syrian town of Azaz, which is under the control of rebels from the Turkey-backed Free Syrian Army factions, could hear the boom of artillery being fired into the region.
"In its second day, #OliveBranchOperation continues to ensure peace and security for our people, protect Syria's territorial integrity and eliminate all terrorist elements in the region," Turkish President Tayyip Erdogan's spokesman Ibrahim Kalin said on Twitter.
"Turkey expects its allies to support its fight against terrorism in all of its forms."