Turkey's president repeated his "Nazi" allegations on Sunday, prompting a retort from the German chancellor and the threat of a blanket ban on Turkish politicians seeking to campaign in Germany.
The row between Turkey and Germany over bans on Turkish politicians campaigning amongst expatriates ahead of an April 16 referendum on expanding presidential powers looks set to continue.
On Sunday, Turkey's President Recep Tayyip Erdogan directly addressed the German chancellor: "Merkel, now you're applying Nazi methods. Against my brothers who live in Germany, and against my ministers and lawmakers who visit there."
"Would this suit the ethics of politics? Your mission is not to support terrorist organisations, but to extradite them."
Merkel responded on Monday, saying "Nazi comparisons by Turkish leaders must stop." Merkel also made clear Germany is considering whether to put a blanket ban on Turkish politicians from speaking to their constituents ahead of the referendum.
"A few days ago, the government made it very clear to Turkey that appearances by Turkish politicians in Germany can only take place on the basis of respect for the principles of German constitutional law. In other words, the government reserves the right to take all necessary measures including reviewing the permissions," the chancellor said.
Previous bans in German cities were implemented by local authorities.
About 1.5 million Turkish nationals eligible to vote in the referendum live in Germany.
"Germany indirectly supports FETO"
Turkey's diplomatic spat with Germany includes different attitudes to Fethullah Gulen, whom Turkey accuses of masterminding the July 15, 2016 coup attempt in Turkey.
On Saturday, German news magazine Der Spiegel published an interview with Germany's intelligence chief Bruno Kahl, who said Ankara had failed to convince Germany that Gulen was responsible for the failed putsch.
"Turkey has tried to convince us of that at every level, but so far it has not succeeded," Kahl said.
Kahl said Germany saw the Gulen movement as a "civil association that aims to provide further religious and secular education."
Kahl also said the German intelligence services did not think the Turkish government was behind the coup: "The coup attempt was not initiated by the government. Before July 15, the government had already started a big purge so parts of the military thought they should do a coup quickly before it hit them too."
Turkey's presidential spokesman Ibrahim Kalin on Sunday responded to Kahl's comments, calling therm "an operation to acquit FETO in Europe. It's an effort to invalidate all the information we have given to Europe and the United States on FETO. It's a sign that from now on, Germany will support FETO just like they support PKK indirectly."
FETO is the acronym Turkey uses for what it calls the Fethullah (Gulen) Terror Organisation.
Kalin said Germany was using FETO members who fled Turkey after the failed coup to its own ends.
"Is it possible that the German intelligence doesn't have information on where these people are, what they are doing and who they are meeting with? ... They are protecting them because these are useful instruments for Germany to use against Turkey."
On July 15, 2016, rogue soldiers commandeered tanks, warplanes, and helicopters, attacking parliament and attempting to overthrow the government, killing 248 people.
Gulen has been living in self-imposed exile in the US since 1999 and denies any involvement in the attempted coup. Ankara is seeking his extradition from the US to stand trial.