Merkel herself - an instinctively cautious leader who rarely rises to provocations - has kept channels of communication open with Turkish President Recep Tayyip Erdogan, most recently holding a bilateral meeting on the sidelines of the Group of 20 summit in Germany two weeks ago.
Relations between the North Atlantic Treaty Organisation partners - already brittle - have been further damaged recently by a dispute over the crackdown by the Turkish authorities that followed a failed coup last July.
The Foreign Ministry issued travel advice which, while short of a formal warning, stated that "people traveling for private or business reasons to Turkey are advised to exercise elevated caution" and to register with the German embassy or consulate even for short stays.
None of the nearly 4,500 extradition demands regarding PKK members issued by Ankara have been responded to by Berlin, Turkish officials have always been saying, which has also been recognized by German Foreign Minister Sigmar Gabriel in early June. "I invite you to prove that we have the "black list" of foreign companies", Erdogan said during his address to foreign investors in Istanbul.
The latest crisis was precipitated by a court order to remand in custody six human rights activists detained on an island off Istanbul. Worse, however, had been observed when Germany and some prominent European countries granted asylum to Gülenists and their sympathizers who fled Turkey in the aftermath of the attempted takeover.
"When you travelled there (the GDR), you knew, if something happens to you, nobody can help you", Mr Schaeuble said.
German-Turkish relations have been souring for over a year since last July's coup attempt in Turkey.
Turkish Prime Minister Binali Yildirim on Friday denied the claims that 68 German firms in Turkey were under investigation. Mr Gulen denies the charges.
"Die Zeit" said the groups probed ranged from carmaker Daimler to chemicals giant BASF to a stall selling doner kebab in the west of Germany.
Erdogan's spokesman, Ibrahim Kalin, meanwhile said: "We strongly condemn statements that German citizens who travel to Turkey are not safe and that German companies in Turkey should have hesitations and concerns".
Turkish officials have sneered that the row is to be seen in the context of the upcoming federal elections in Germany, where Gabriel will play a key role. As Yıldırım outlined in a statement on July 21, it would be no exaggeration if one categorized the relationship between Turkey and Germany as unique and as one of the most special in worldwide relations. That overhaul "covers all sectors, including defence exports policy", the economy ministry said.
Germany, home to three million people with Turkish roots, said it would review Turkish applications for arms projects.
Tensions between Ankara and Berlin spiked in the spring over Germany's refusal to allow Mr Erodgan's party to hold rallies in the campaign for the referendum on expanding the president's powers.
Before the failed coup and the massive crackdown which followed, members of Gulen's movement were accused of running a parallel shadow government after gaining control of state institutions which included the military and the judiciary.
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