ISTANBUL — Relations between Turkey and Germany, Ankara's key European ally, appear headed for a rough ride following revelations that the German Federal Intelligence Service (BND) has been spying on Turkey since at least 2009.

Asserting that “some principles” should always be respected in the international arena, especially between allies, Foreign Minister Ahmet Davutoglu demanded answers from Turkey's NATO partner on Aug. 18. “We are faced with a situation that must definitely be explained,” he said.

The alleged surveillance was made public by the German news magazine Der Spiegel on Aug. 16. While the government of Chancellor Angela Merkel has not publicly confirmed the allegations, German media have quoted government officials as justifying the surveillance, arguing that events in Turkey have implications for Germany’s national security given that roughly 3 million Turks and citizens of Turkish descent live in Germany.

Turkish media reported that the BND probably listened in to phone calls by Turkish officials via satellite. The reports speculated that German agents had been especially interested in details about efforts by Turkey to end the 30-year-old insurgency by the Kurdistan Workers Party (PKK) and about Ankara’s troubled relations with Israel. No official statement about the nature or the targets of the surveillance has been made available.