Turkey on July 20 told Germany that bilateral ties could not be founded on "blackmail and threats" after the German foreign minister vowed steps that would limit investment in an escalating dispute.
"Our relations cannot be pursued based on blackmail and threats but through internationally accepted norms and principles," the Turkish Foreign Ministry said in a statement, accusing German Foreign Minister Sigmar Gabriel of a "one-sided and distorted approach."
Gabriel had said earlier on July 20 that Germany would review state guarantees for foreign investment in Turkey and urge businesses against putting their money in the country.
The German Foreign Ministry also warned it could no longer guarantee its citizens' safety in the country because of the current wave of arrests.
Relations between the two NATO allies have been deteriorating for more than a year, particularly since last July's coup attempt in Turkey. Twenty-two German citizens have been taken into custody in the ensuing crackdown. Nine are currently in jail, including human rights activist Peter Steudtner and journalists Deniz Yucel and Mesale Tolu.
The immediate trigger of Germany's anger was a Turkish court's decision July 18 to jail Steudtner and other activists, including Amnesty International's Turkey director and a Swedish IT trainer, who were detained in a July 5 security raid on a hotel on the island of Buyukada, where they were attending a digital-security workshop.
Amnesty says Steudtner was providing training on nonviolence. It's unclear which terror group the six are accused of aiding.
Germany is furious because Steudtner has no previous links to Turkey and yet he was accused of links to terror, which Berlin says are far-fetched. Gabriel said the case "shows that German citizens are no longer safe from arbitrary arrests."
However, the Turkish Foreign Ministry accused Germany of putting those "on trial and German guests, who come to our country as tourists, on the same scale."
It said that there had been a "serious confidence crisis" in Turkish-German relations in recent months due to what the ministry called Germany's "approach of double standards."
Speaking on a visit to Turkish-backed breakaway northern Cyprus, Foreign Minister Mevlut Cavusoglu warned Berlin that Turkey could hit back at Germany, without giving details.
The countries also have clashed over Turkey blocking German lawmakers' visits to German troops serving in the campaign against the Islamic State group at two Turkish air bases and over authorities' refusal to allow rallies by Turkish government politicians in Germany.
With reporting by Reuters, AP, AFP, dpa, and BBC