“The Turkish military, together with the Free Syrian Army, will cross the Turkish-Syrian border shortly. YPG militants have two options: They can defect or we will have [to] stop them from disrupting our counter-ISIS efforts,” Turkish government spokesman Fahrettin Altun said in a tweet on Tuesday.

The warning comes on the heels of a minor American military pullout from the region, clearing the way for the Turkish operation. Airstrikes on Kurdish positions along the Syrian-Iraqi border were reported less than a day after the move, as Ankara prepared for its incursion.Turkey has long complained about the presence of Kurdish militia fighters along the border in northern Syria, arguing they are allied with the Kurdistan Workers Party (PKK), an armed group in Turkey which has feuded with the government intermittently for decades.