Turkish ground troops entered Syria on Sunday to push an offensive against Kurdish militia as rocket fire hit a border town in apparent retaliation.
Turkey on Saturday launched operation "Olive Branch" seeking to oust from the Afrin region of northern Syria the Peoples' Protection Units (YPG) which Ankara considers a terror group.
But the campaign risks further increasing tensions with Turkey's NATO ally the United States -- which has supported the YPG in the fight against Islamic State jihadists -- and also needs at least the tacit support of Russia to succeed.
Prime Minister Binali Yildirim said troops crossed into the YPG-controlled region in Syria at 0805 GMT, the Dogan news agency reported.
Turkish artillery and warplanes pounded YPG sites around Afrin and total of 153 targets, including YPG refuges and weapons stores have now been hit, according to the army.
The state-run Anadolu news agency said the Turkish troops, whose number was not specified, were advancing alongside forces from the pro-Ankara rebel Free Syrian Army (FSA) and were already five kilometres (three miles) inside Syria.
An AFP correspondent on the southwestern edge of the Afrin region saw a warplane bombing the western outskirts of the area early on Sunday.
A small unit from a Turkish-backed rebel group was manning a monitoring point on a hilltop overlooking several Kurdish-controlled villages below.
'No need to worry'
The operation is Turkey's second major incursion into Syria during the seven-year civil war after the August 2016-March 2017 Euphrates Shield campaign in an area to the east of Afrin against both the YPG and IS.
The army said IS was also being targeted in this operation although it no longer has any major presence in the Afrin area.
President Recep Tayyip Erdogan had repeatedly vowed that Turkey would root out the "nests of terror" in Syria of the YPG, which Ankara accuses of being the Syrian offshoot of the Kurdistan Workers' Party (PKK).
The PKK, which has waged a rebellion in the Turkish southeast for more than three decades, is regarded as a terror group not just by Ankara and but also its Western allies.
Afrin is an enclave of YPG control, cut off from the longer strip of northern Syria that the group controls to the east extending to the Iraqi border. Turkey wants the YPG to retreat east of the Euphrates River.
Yildirim was quoted as saying that the Turkish forces aimed to create a security zone some 30 kilometres (18 miles) deep inside Syria.
The YPG said that after the first strikes on Saturday 10 people were killed, including seven civilians. The Turkish army said there were casualties but insisted they were all members either of the YPG or the PKK.
A YPG spokesman claimed that the Turkish forces had sought to enter Afrin "but we blocked the attack".
In a sign of the risks to Turkey, four rockets fired by the YPG hit the border town of Kilis early Sunday, damaging one building and lightly wounding a woman.
"No one lost their life," Kilis governor Mehmet Tekinarslan said, quoted by Dogan. "They can fire one rocket at us and we will fire 100 back. There is no need to worry."
'Russian green light?'
Turkey risks entering a diplomatic minefield with its action in Syria and the foreign ministry lost no time in inviting the ambassadors of all major powers to be briefed on the offensive.
The ministry said it had even informed Damascus through its Istanbul consulate. But the Syrian regime, which is at odds with Turkey, strongly denied this, denouncing the operation as a "brutal Turkish aggression".
There was no immediate comment from the United States on the offensive but ahead of its launch, a senior State Department official had raised concerns it risked being harmful for security in the region.
But even more crucial is the attitude of Russia, which has a military presence in the area and is also working with Turkey on a drive to end the civil war.
The Russian foreign ministry voiced concern and urged Turkey to show restraint. And the defence ministry said its troops were withdrawing from the Afrin area to ensure their security and prevent any "provocation".
Timur Akhmetov, Ankara-based researcher at the Russian International Affairs Council, told AFP that Russia appeared to have given the "green light" to the operation but made clear it should not lead to destabilisation elsewhere.
"I don't think Russia will agree to let Turkey occupy the whole Afrin region and insists on keeping the Syrian government in charge," he added.