"The Turkish government needs to immediately and directly hear the German government's outrage and incomprehension as well as its crystal-clear expectations in the case of Peter Steudtner, and this time without diplomatic niceties," foreign ministry spokesman Martin Schaefer said.
He said the envoy was told "in no uncertain terms" that the detention of Steudtner and five other activists including Amnesty International's Turkey director was "incomprehensible and also unacceptable".
Berlin demanded Steudtner's "immediate release" as well as consular access.
A Turkish court yesterday ordered six human rights activists including Amnesty International's Turkey director to remain in custody for allegedly aiding a "terror" group, a charge Schaefer branded "absurd".
Chancellor Angela Merkel has criticised the detention of the activists as "unjustified" and pledged to "advocate for (Steudtner) on all levels".
Schaefer said Foreign Minister Sigmar Gabriel would interrupt his summer holidays and return to Berlin over the "dramatic escalation in Turkish actions" to determine which "further measures" could be taken.
Relations between Turkey and Germany, home to three million ethnic Turks, have been badly strained, particularly since the failed coup attempt a year ago against Turkish President Recep Tayyip Erdogan.
One dispute centres on Deniz Yucel, a German-Turkish journalist with the Berlin-based newspaper Die Welt, who was imprisoned by Turkey on terror charges earlier this year.
Schaefer also mentioned the ongoing detention of German reporter Mesale Tolu, who has been in Turkish custody since April.
The NATO allies have also clashed over thwarted visits by German lawmakers to German troops stationed at Turkish bases.
Last week Turkey asked legislators to postpone a scheduled visit to a NATO base in Konya, the German foreign ministry said, expressing "regret" about the decision.
Merkel's spokesman Steffen Seibert said Wednesday that in the current context, any progress on Turkey's bid to join the EU was "not on the cards".
He pressed Ankara to consider what relationship it wanted to have with the 28-member bloc given the outstanding "questions about the rule of law" and democratic values in the country.
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