Turkish war planes have launched air strikes on Kurdish fighters in northern Syria in a move likely to raise tensions with the US.
Turkey wants to oust the YPG militia, which it calls a terrorist group, from Afrin region on its southern border.
The US-backed YPG says at least nine people were killed in the air strikes.
Russia has moved its troops away from Afrin, saying it is concerned but will not interfere. Syria denounced Turkey's "aggression" and "brutal attack".
Turkey had been shelling the area for two days, ahead of its declaration of a military operation on Saturday.
The YPG has been a key part of the battle against IS in Syria, and has been supported by the US.
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.
Turkey's military plans seem to have been accelerated by an announcement from the US that it will help the SDF alliance 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, reports BBC.