NATO allies Ankara and Berlin have been locked in a new dispute over the past week after several local authorities in Germany blocked rallies by Turkish ministers.
The row is the latest in a long list of problems that have plagued relations and comes just after Ankara's arrest of a journalist with the German daily Die Welt that sparked consternation in Berlin.
On Sunday, President Recep Tayyip Erdogan raised the stakes when he told a rally in Istanbul that the blocking of public appearances by his ministers was "not different from the Nazi practices of the past".
German Foreign Minister Sigmar Gabriel, after hosting his Turkish counterpart Mevlut Cavusoglu for talks, said he had "made clear that comparisons between the Nazi era and the cancellation of rallies or the rule of law in Germany are unacceptable".
"Both sides have a responsibility to simply not cross certain red lines, and comparisons to Nazi Germany are one of them," said Gabriel.
Despite Berlin and Ankara's deep differences on a host of issues, "there is no alternative to dialogue because that is the only way we can return, step-by-step, to a normal and friendly relationship," he added.
Speaking separately after their meeting, Cavusoglu acknowledged that poor relations were in neither side's interests.
He said he would host Gabriel for a new round of talks in Turkey "as soon as possible".
At the same time, he told Germany to decide "if Turkey is a friend or not".
'Please don't preach'
Turkish ministers have been streaming into Germany to campaign in favour of an April referendum on expanding Erdogan's powers.
The ministers are anxious to tap into Germany's Turkish community with its 1.4 million people who are eligible to vote -- the fourth largest electoral base after Istanbul, Ankara and Izmir.
Cavusoglu had himself travelled to the northern German city of Hamburg where he addressed some 200 supporters at the residence of the Turkish consul on Monday evening.
"Please, don't give us lessons in human rights and democracy," the minister said, accusing Germany of failing to practice what it preached in reference to the scrapped rallies.
Erdogan had also warned Berlin not to hinder him from making an appearance if he wished.
Asked if the Turkish president might make a campaign appearance, the German foreign ministry said: "There is to date no official request for a visit to Germany by Erdogan."
Separately, Zurich authorities said Wednesday they have asked the Swiss government to cancel a planned rally by Cavusoglu on Sunday because of security concerns.
And Austrian chancellor Christian Kern has called for a ban on Turkish politicians from politically campaigning across the EU.
'Part of our country'
Germany and Turkey have a special relationship due to the large community of Turks who have settled in Europe's biggest economy, the legacy of a "guest worker" ("Gastarbeiter") programme dating to the 1960s and 70s.
But those ties have been put to the test in the past year over differences on issues surrounding human rights and press freedom, particularly since last July's failed coup in Turkey aimed at ousting Erdogan.
Berlin has emerged as a strident critic of Ankara's vast crackdown in the aftermath of the putsch, which has seen more than 100,000 people arrested, suspended or sacked for alleged links to the plotters or to Kurdish militants.
Ankara has in turn accused Berlin of harbouring "terrorists" and failing to respond to requests to hand over suspects from the coup as well as Kurdish militants who it believes are members of the outlawed PKK group.
With the latest feud rumbling on, Germany's domestic spy agency on Wednesday voiced concern that tension between nationalist Turks and PKK supporters could escalate.
"We have seen for a long time that conflicts in Turkey also have an impact on the security situation in Germany," said Hans-Georg Maassen, who heads the spy agency.
"The dividing lines between the various camps in Turkey are mirrored in Germany," he added.
Gabriel meanwhile sought to prevent an 'us-versus-them' sentiment from building up among Turks in Germany.
In a special message to the community, Gabriel said: "When there are political differences, Germany must not allow political disputes to be imported from Turkey."
Immigrants "from Turkey have played an incredible part in building prosperity in our country," he said.
"You are a part of our country."
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