German press reports suggest the country’s BND intelligence agency spied on Turkey for years.
Der Spiegel magazine said Berlin had identified Ankara as a top surveillance target, citing a 2009 government document.
Turkey has summoned Germany’s ambassador over the allegations.
“This is totally unacceptable. All necessary measures will be taken legally,” said Turkey’s Energy Minister Taner Yildiz.
“The Turkish foreign ministry is working on the issue. Turkey is a state of law and we will respond within the framework of law both internationally and nationally,” Yildiz added.
Turkey’s largest trading partner in the EU is Germany, home to some three million Turks.
German Chancellor Angela Merkel is no stranger to spying. She was furious when she discovered her mobile phone had been tapped by the US National Security Agency (NSA), after which she famously said:“Spying among friends is not at all acceptable.”