(CNN) German Chancellor Angela Merkel has demanded the release of German citizens detained in Turkey, describing their imprisonment as "unjustified" on Tuesday.

Relations between Berlin and Ankara have been in a downward spiral since last summer, when a failed coup against Turkish President Recep Tayyip Erdogan sparked a crackdown on civil liberties and mass arrests of the political opposition, activists and journalists, including German citizens.

Germany has accused Erdogan of attempting to silence his critics at home and abroad. Erdogan, in turn, has called on voters in Germany to reject the country's biggest parties in next month's election.

JUST WATCHED Wife of imprisoned German-Turkish journalist speaks out Replay More Videos ... MUST WATCH Wife of imprisoned German-Turkish journalist speaks out 03:04

Speaking at her annual summer news conference on Tuesday, Merkel said Turkey's jailing of German citizens was further damaging already fraught ties between the two countries.

"We must see how things develop but we are calling now, very clearly, for the release of those who are imprisoned," she said.