Turkish troops fired on U.S.-backed Kurdish guerrilla fighters in northern Syria on Thursday, highlighting the complications of an incursion meant to secure the border region against both Islamic State and Kurdish advances.

Syrian rebels backed by Turkish special forces, tanks, and warplanes entered Jarablus, one of Daesh ‘ISIS”s last strongholds on the Turkish-Syrian border, on Wednesday.

But President Tayyip Erdogan and senior government officials have made clear the aim of “Operation Euphrates Shield” is as much about stopping the Kurdish YPG guerrillas seizing territory and filling the void left by Islamic State as it is about eliminating the ultra-hardline Islamist group itself.

Turkey, which has NATO’s second biggest armed forces, demanded that the YPG retreat to the east side of the Euphrates river within a week. The Kurdish guerilla had moved west of the river earlier this month as part of a U.S.-backed operation, now completed, to capture the city of Manbij from Daesh.