Turkey's president has compared German officials to Nazis, in the latest escalation in a war of words.
Recep Tayyip Erdogan hit out after German authorities cancelled rallies designed to woo ethnic Turkish voters in Germany ahead of a key referendum.
"Your practices are not different from the Nazi practices of the past," Mr Erdogan said.
German Justice Minister Heiko Maas said the comments were "absurd, disgraceful and outlandish".
About 1.4m Turks in Germany can vote in the April referendum, which could give Mr Erdogan sweeping new powers.
Voters will be asked whether they back a new constitution, which would transform the country from a parliamentary republic into a presidential one.
That would give Mr Erdogan, as president, new powers over the budget and the appointment of ministers and judges, as well as the power to dismiss parliament.
Speaking to German broadcaster ARD, Mr Maas said banning Mr Erdogan from visiting Germany or breaking off diplomatic ties would "play straight into the arms of [Russian President Vladimir] Putin, which no one wants".
Russia and Turkey have steadily improved ties as both countries' relations with the EU have cooled.
The deputy leader of Chancellor Angela Merkel's Christian Democratic Union party, Julia Kloeckner, said Mr Erdogan was "reacting like a stubborn child who can't get his own way".
Mr Erdogan has come under intense international criticism amid a far-reaching crackdown on his opponents since a failed military coup last year. Dozens of Turkish journalists and writers have been arrested, and a large number of diplomats and soldiers have sought asylum elsewhere.
Against this background, Turkey is targeting the millions of expatriate voters eligible to cast a ballot in the controversial poll - including the 1.4m in Germany.
However, officials have withdrawn permission for rallies in Gaggenau, Cologne and Frechen.
The Gaggenau authorities said there was insufficient space for the rally, while Cologne officials said they had been misled about the purpose of the event.
Addressing the German authorities at a rally in Istanbul on Sunday, Mr Erdogan said: "I thought it's been a long time since Germany left [Nazi practices]. We are mistaken."
His comments were in reference to the German Nazi party, which Adolf Hitler led to power from 1933 and 1945. The Nazis were responsible for the mass murder of six million Jews and millions of other people during, World War II.