Turkey’s military says it has returned fire after a cross-border attack by Syrian Kurdish forces.

A statement on Wednesday said the People’s Protection Units, or YPG, fired on Turkish territory overnight with anti-aircraft weapons from Syria’s Afrin region. Turkish artillery units returned fire, destroying the “detected targets.”

The YPG is the main component of the Syrian Democratic Forces, a U.S.-backed militia that is battling the Islamic State (IS) group in the extremists’ de facto capital, Raqqa. Turkey views the YPG as an extension of the Kurdish insurgency raging in its southeast.

Turkey was angered by a U.S. decision last month to arm the Syrian Kurds, fearing the weapons will end up in the hands of Kurdish rebels in Turkey.