Turkish ground troops have crossed into northern Syria as part of Ankara's campaign to rid the border area of Kurdish fighters, Turkey's PM says.
Binali Yildirim said the aim was to establish a 30km (19-mile) "safe zone" deep inside Syria, Turkish reports say.
Turkey launched its operation against Kurdish YPG militia in the Afrin region with air strikes on Saturday.
It risks increasing tensions with the US, which had supported the YPG in the fight against Islamic State jihadists.
The YPG has denied Turkish troops were on Afrin soil, saying they had "repelled" the attacks and forced them to retreat.
The Turkish military says it has so far taken out 153 targets belonging to Kurdish militants, while Kurdish rockets hit the border town of Kilis overnight.
The YPG (Syrian Kurdish People's Protection Units) is part of a US-backed alliance with a number of ethnic Arab militias called the Syrian Democratic Forces (SDF). The SDF has said it will have no choice but to defend itself if attacked.
On Saturday, Turkey announced that an air and ground campaign, dubbed "Olive Branch", had been launched at 14:00 GMT, targeting the YPG and Islamic State (IS) group jihadists.
The operation would be carried out "with respect for Syria's territorial integrity", it added.
Pro-Turkey rebels, known as the Free Syrian Army, have also begun moving into the area, according to the state-run Anadolu news agency.
The YPG said the strikes had killed at least six civilians and three fighters, with another 13 civilians wounded. One of the fighters belonged to the YPG and the other two were from its all-female affiliate, spokesman Birusk Hasaka said.
Ankara also said there were casualties, but they were all Kurdish militants.
Prime Minister Yildirim was quoted by Turkish media as saying ground forces joined the operation by crossing into YPG-controlled territory on Sunday morning.
He gave no details about the 30km security zone troops are aiming to set up.
The YPG denied the reports. "All the Turkish military's ground attacks against Afrin have been repelled so far and they have been forced to retreat," said Nouri Mahmoudi, a spokesperson for the group.
Turkey's military has been shelling the region since Thursday, a move which it said was in response to fire coming from the area.