Turkish President Recep Tayyip Erdogan said on Sunday that Germany has no right to interfere in Turkey's domestic affairs.

“Turkey is a social democratic state based on the law and no one has the right to interfere in its internal affairs,” said Erdogan before leaving on a trip to the Gulf.

Several German nationals are among those being held in Turkey. Berlin has warned its citizens that their safety cannot be guaranteed [in Turkey] and that consular access is not assured in case of arrest.

Throwing away any pretence at diplomatic nuance, German Foreign Minister Sigmar Gabriel on Thursday had warned German firms against investment in Turkey and spoke of an “overhaul” of the entire relationship.

In a letter published in German and Turkish in daily newspaper Bild, Gabriel had assured the three million German citizens of Turkish descent that "it is clear that you, people of Turkish roots in Germany, belong here with us, whether you have a German passport or not."

Addressing Gabriel's comments, he said, “We (Turkey and Germany) are together in NATO. We are in negotiations to join the European Union (EU). So the strategic partnership between us is nothing new. We have been partners for a long time. No step should be taken to overshadow this partnership."

Turkey remains under a state of emergency imposed days after last year's failed coup, which critics claim is being used to go after any opponent of Erdogan. The authorities, on the other hand, claim that the state of emergency is important for public security.

The latest crisis was precipitated by the order of a Turkish court to remand in custody a group of human rights activists detained on an island off Istanbul, including Amnesty International's Turkey director Idil Eser and Berlin-based activist Peter Steudtner.

Berlin was already furious over the jailing of Deniz Yucelin, Turkey's correspondent for Die Welt newspaper, in February, who Erdogan has personally denounced as a “terror agent”.

Meanwhile, Turkey has accused Germany of not doing enough to deal with Kurdish militants and suspects from the failed coup who have taken refuge on its soil.