Germany raised the possibility on Wednesday of suspending European Union aid payments to Turkey after summoning Ankara's ambassador to Berlin to protest over the arrest of six human rights activists including a German citizen.
The moves mark a further escalation of tensions between NATO allies Germany and Turkey, who are at loggerheads over a wide range of issues.
This month, Turkey arrested rights activists including Amnesty International's Turkey head Idil Eser and German citizen Peter Steudtner on terrorism charges, which Berlin has labelled "absurd".
"Unfortunately we have constant cause to talk to Turkey about civil and press freedoms," government spokesman Steffen Seibert told a news conference. "We think it is important to review aid in light of the latest developments."
Seibert was referring to a package of assistance totalling three billion euros that the EU has promised to Turkey to help stem flows of migrants, mostly from conflict zones in the Middle East, into Europe.
The deal has helped to sharply reduce the number of arrivals and has been especially welcomed by Germany, the preferred destination for the migrants, and Chancellor Angela Merkel.
German Foreign Minister Sigmar Gabriel cut short his holiday to deal with the latest rise in tensions with Turkey, whose ability to rekindle Europe's refugee crisis could harm Merkel's prospects in a national election due in September.
The six rights activists are among tens of thousands of Turks swept up in a crackdown that followed last year's failed coup against Turkish President Tayyip Erdogan. Many have accused Erdogan of using the dragnet as an excuse to purge opponents.
"Claims of terrorist links are absurd," the German foreign ministry said in a statement demanding consular access to its citizen and the release of all the activists. "It is appalling to accuse Amnesty and other rights organisations of terrorism."
Bilateral relations have already been tested by, among other issues, Turkey's arrest of Deniz Yucel, a Turkish-German dual national who is a correspondent for the German newspaper Die Welt, on similar terrorism and propaganda charges.
Ankara is seeking the extradition of army officers whom it accuses of involvement in the coup attempt. Berlin has declined to return the men, who have sought asylum in Germany