The incident came as Turkey carries out strikes targeting Kurdish forces in the area following a suicide bombing on Sunday in Ankara that was claimed by the Kurdistan Workers' Party (PKK), which is considered a terror group by Turkey and its Western allies.

US forces observed drones carrying out air strikes in northeast Syria on Thursday morning, some of them inside a "restricted operating zone" (ROZ) near Hasakah, about a kilometer (less than a mile) from American troops, Pentagon spokesman Brigadier General Pat Ryder told journalists.

A few hours later, a Turkish drone returned to the ROZ heading toward US forces.

"US commanders assessed that the UAV, which was now less than a half kilometer from US forces, to be a potential threat and US F-16 fighters subsequently shot down the UAV in self defense," he said, using an abbreviation for unmanned aerial vehicle.

The United States has approximately 900 troops deployed in Syria as part of international efforts to combat the Islamic State (IS) group, and carries out frequent raids targeting the jihadists.