Germany says it will keep calling for access to a German-Turkish journalist detained in Istanbul since January on charges of terrorist propaganda and incitement to hatred.

Foreign Ministry spokesman Martin Schaefer told reporters in Berlin on Monday that "everyone involved, including in Turkey, knows that the emblematic case of Deniz Yucel has great political and symbolic importance for German-Turkish relations."

He acknowledged that since Yucel is also a Turkish citizen, Turkey has no obligation to grant Germany consular access, but said Turkey's prime minister had said German diplomats would be allowed to visit him.

Schaefer said: "We won't stop demanding what we've been promised. But Turkey is a sovereign state that, whether we like it or not, has the right to allow this or not."