Turkey’s military says its war planes have launched air strikes on Kurdish positions in northern Syria, in a move likely to cause tensions with the US.
It is seeking to oust Kurdish fighters from Afrin region, held by them since 2012.
Some of these forces helped the US to fight the Islamic State group.
There are also reports of pro-Turkish rebels advancing on the city.
Turkey has previously threatened a full military operation against these Kurds – which it considers a terrorist group.
President Recep Tayyip Erdogan has also threatened to strike at the city of Manbij – some 100km away from Afrin in another Kurdish area – as a next target.
Meanwhile, Russia – a key military figure in the region – said it will not interfere in any conflict in Afrin.
Syria has 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 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 been threatening to clear Kurdish fighters from both Afrin and Manbij for several months.
Its 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 Erdogan called the border force a “terror army”.
Speaking at a meeting of his ruling AKP party, Mr Erdogan once again linked the YPG and other Syrian Kurdish groups to the Turkish PKK, an insurgent group considered a terrorist organisation by Turkey.
The Turkish military started artillery shelling of the Afrin region on Thursday, saying it was responding to fire coming from the area.