Turkish war planes have launched air strikes on Kurdish positions in northern Syria, in a move likely to cause tensions with the US.
Turkey wants to oust these Kurdish fighters from Syria’s Afrin region, which lies near its southern border.
It considers them a terrorist group. But some were US allies in the battle against the Islamic State group.
Turkey had been shelling the area for two days, ahead of its declaration of a military operation on Saturday.
Russia – a key military figure in the region – says it is concerned by the development, and has relocated some of its troops based in the region. Officials earlier said Moscow would not interfere in the conflict.
Syria has previously warned against any operation and said it would shoot down Turkish planes.
The Kurdish YPG (Syrian Kurdish People’s Protection Units) has been a key part of the battle against the group calling itself the Islamic State (IS) in Syria, and has been backed by the United States.
Turkey, however, believes the group has links to the banned Kurdistan Workers Party (PKK), and has for several months been threatening to clear Kurdish fighters from Afrin and another city, Manbij, which lies 100km away. The Kurds have held Afrin since 2012.
Turkey’s military plans seem to have been accelerated by an announcement from the US that it will help the Syrian Democratic Forces (SDF) – an alliance against IS of which the YPG is a member – build a new “border security force” to prevent the return of IS.
The YPG and SDF deny any terrorist links – a claim backed by the US government.
But President Recep Tayyip Erdogan called the border force a “terror army”.
Disagreement over the Kurdish fighters has created a sharp division between the Nato allies.
The US state department has appealed for calm, and attempted to downplay portrayals of a new “border force”, instead characterising the new development as security training.
“We do not believe that a military operation… serves the cause of regional stability, Syrian stability, or indeed Turkish concerns about the security of their border,” it said.
On Saturday, the Turkish army announced that a new campaign, dubbed “Olive Branch”, had launched at 14:00 GMT, targeting the YPG and IS 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, also began moving into the area, according to the state-run Anadolu news agency.
A spokesperson for the YPG told Reuters a number of people had been injured in the strikes so far, but it is not yet clear how many.
Turkey’s military has been shelling the area since Thursday, a move which it said was in response to fire coming from the area.
On Saturday, the SDF accused Turkey of using the bombardment as a smokescreen ahead of launching an offensive. A spokesperson for the group told Reuters news agency that it would have no choice but to defend itself if attacked.
Rizan Habou, of the Syrian Democratic Council in Afrin, told BBC Arabic that residents were seeking shelter.
“When the villages in Afrin are shelled, the civilians [including] women and children are forced to leave their houses and go to the relatively safer surrounding open space and farmland till the shelling stops,” he said.
“Afrin is isolated from two other self-declared Kurdish autonomous cantons – Kobani and Jazira.
Turkish-backed rebel forces took over a 100km (60-mile) area separating the territories after driving out IS in 2016 – so driving out Kurdish fighters would significantly expand Turkey’s area of control in the region.
Russia’s foreign ministry says it is concerned by news of the offensive, and is urging restraint.
Russian senator Frants Klintsevich – who is the deputy chairman of the defence and security committee – earlier told Interfax news agency that Moscow will only respond if Russian bases in Syria are threatened.
He said that Russia has been placed in a difficult situation, as it has “good relations with both Damascus and Ankara”.
Turkey’s military and intelligence chiefs had been trying to get Russia’s agreement to allow Turkish planes to use the Russian-controlled airspace above Afrin.
Such consent is seen as essential for any Turkish operation. Moscow is a key ally of Syrian President Bashar al-Assad and has a contingent of soldiers at the airport in the centre of Afrin.
It is not yet clear if Russia’s claim that it will not interfere precludes allowing use of its airspace.
Turkish Foreign Minister Mevlut Cavusoglu has reportedly discussed the military offensive with US Secretary of State Rex Tillerson, as did Turkey’s chief of military staff with his US and Russian counterparts.
However, no details of the conversations have been provided.