Turkish President Recep Tayyip Erdogan continues to test the patience of Chancellor Angela Merkel. Recent comparisons to Nazi Germany by the president and other Turkish ministers are now followed by reports of Turkish espionage on German soil, deepening a rift between the NATO partners.

Turkish President Recep Tayyip Erdogan’s long and grasping arm has extended to Germany, but probably not with the result he would have liked.

Turkish spy agents have identified hundreds of alleged supporters in Germany of the exiled Muslim cleric Fethullah Gülen, accused by the president of orchestrating the failed military coup last July. Not only have Turkish intelligence agents been spying on their own countrymen in Germany; they have also tried to sign up the German secret service as collaborators, according to information obtained by two German public broadcasters and the Süddeutsche Zeitung newspaper.

On the sidelines of the Munich Security Conference earlier this month, Hakan Fidan, head of Turkey’s national intelligence agency MIT, handed his German counterpart Bruno Kahl a list of more than 300 people and 200 various clubs, schools and other institutions with alleged links to Mr. Gülen, according to the media report.

Mr. Fidan hoped the German foreign intelligence service, the BND, would provide assistance. But the opposite has happened.

Mr. Kahl promptly handed over the material to Germany’s domestic intelligence agency and a number of other agencies, including the federal criminal police office and the attorney general, to weigh legal action. It has even informed some of those being monitored.

Informing the Germans of the espionage activities may have backfired, but they've still sparked sharp responses from the country's politicians.

Regardless of what you think of the Gülen movement, German law applies here and citizens who live here won’t be spied on by foreign states. Thomas de Maiziére, German Interior Minister

German Interior Minister Thomas de Maiziére, speaking to reporters on Tuesday, called the espionage as totally unacceptable. “Regardless of what you think of the Gülen movement, German law applies here and citizens who live here won’t be spied on by foreign states,” he said.

Germany is already investigating possible spying by Turkish imams in the country.

Boris Pistorius, the interior minister of Lower Saxony, said at a press conference that the “intensity and ruthlessness of investigating people abroad is remarkable.” He added that there was “no evidence that Gülen supporters in Germany had anything to do with the attempted putsch.”

Mr. Pistorius accused the Turkish government of having an “almost paranoid fear of conspiracy” and of trying to silence its critics.

Mr. de Maizière, Foreign Minister Sigmar Gabriel and Integration Commissioner Aydan Özoguz have invited Turkish political representatives in Germany to a meeting in the foreign ministry on Wednesday. The discuss will center on how the difficult relations between Berlin and Ankara are affecting their political activities.

The clashes with German politicians and authorities come ahead of a referendum next month in Turkey that proposes to significantly expand Mr. Erdogan’s presidential powers. More than 1.4 million Turks in Germany are eligible to vote on the referendum and can do so over the next several days.

The referendum has deeply strained relations between Germany and Turkey, not just because of German concerns about the political impact of the vote in Turkey, but because of a series of sharp barbs surrounding it. Mr. Erdogan has repeatedly accused Germany of "Nazi tactics" after some German cities banned rallies in favor of the referendum by cabinet members, citing security concerns.

Some of Germany's politicians also haven't been shy about injecting their own views into the referendum. Norbert Lammert, the president of Germany’s lower chamber of parliament, the Bundestag, said the referendum was about “transforming an undoubtedly fragile but democratic system into an authoritarian system – and this second coup may be successful.”

John Blau is a senior editor with Handelsblatt Global. To contact the author: [email protected]