LONDON (NYTIMES) - Wading into an escalating diplomatic feud, the European Union warned Turkey on Monday (March 13) that a constitutional amendment to drastically strengthen the president's powers might harm the country's long-standing bid to eventually join the bloc.

EU officials also urged Turkey to avoid inflammatory rhetoric like the recent declarations by President Recep Tayyip Erdogan, who said that Germany and the Netherlands had demonstrated Nazi-like behaviour in blocking Turkish officials from campaigning in those countries.

Membership talks with Turkey have been moribund, but the prospect of Turkey's eventually joining the European Union has also long been dangled as an enticement to closer ties.

The warning, delivered in a statement by two officials of the European Commission, the EU's executive arm, was the bloc's strongest collective response yet to a sustained verbal assault from Turkey.

The comments have infuriated officials in Germany and the Netherlands, where Turkish migrants and their families make up a sizable minority.

"It is essential to avoid further escalation and find ways to calm down the situation," said the statement by officials Federica Mogherini of Italy, the bloc's foreign policy chief, and Johannes Hahn of Austria, the European commissioner for negotiations on expanding membership in the 28-nation bloc.

"The European Union calls on Turkey to refrain from excessive statements and actions that risk further exacerbating the situation."

Also on Monday, the Venice Commission, which advises European leaders and studies democracy under the rule of law, issued a report that said the referendum risked giving an excessive concentration of power to the presidency, harming checks and balances and judicial independence. The report also expressed concern that the change would take place during a state of emergency that was declared after a failed coup attempt against Erdogan in July.

On March 5, Erdogan accused Germany of engaging in Nazi-like behaviour after the government of Chancellor Angela Merkel, citing security concerns, cancelled rallies at which Turkish government ministers had planned to urge Turkish voters living in Germany to vote yes in the April 16 referendum.

The remarks infuriated officials in Germany, where awareness of the crimes of the Nazi state is enshrined in school curriculums and public commemorations.

On Sunday, Erdogan said that "Nazi remnants" were in the Dutch government, after it refused to allow two of his ministers to hold rallies in the Netherlands.

His inflammatory comments came at an acute time: Voters go to the polls in the Netherlands on Wednesday to elect a new Parliament, and the Freedom Party led by far-right, anti-immigration politician Geert Wilders is expected to make a strong showing, possibly coming in first place.

Erdogan's fiery remarks continued to ripple through European politics on Monday.

At an economic meeting in Munich, Merkel said she had told Parliament last week that she "rejected all kinds of rhetorical comparisons with Nazism in Germany made by Turkish personalities."

"This rejection also fully applies, of course, to friendly countries such as the Netherlands," she said, adding that "these comparisons are completely misleading" and "trivialize the sufferings" of the victims of Nazism.

"Especially with respect to the Netherlands, which suffered under National Socialism, that is completely unacceptable," Merkel said. "And that is why the Netherlands has my full support and solidarity."

Nazi Germany occupied the Netherlands from 1940 until the end of War II in 1945.

At a news conference in Rotterdam, the Netherlands' prime minister, Mark Rutte, said he did not think that Turkey was trying to interfere in his country's elections, but he added that his government would not be intimidated by bluster.

"Turkey is a powerful country, but the Netherlands is also a powerful country," Rutte said.

The government, he said, had been prepared to allow the Turkish foreign minister, Mevlut Cavusoglu, to meet with "a limited group of Turkish people in one of the official Turkish residences," but that while it was being arranged, "we saw on CNN Turk that he was threatening the Netherlands with sanctions."

"We tried to de-escalate the situation," Rutte said, but Cavusoglu complicated matters by citing the Dutch elections as the reason he was denied permission to fly to Rotterdam. "It's very clear that that is not true," Rutte said. "We will still try to de-escalate, but of course to de-escalate, it takes two to tango."

Rutte said he was grateful for expressions of sympathy from Merkel, from EU officials and from the French foreign minister, Jean-Marc Ayrault, among others.

Turkish officials, nonetheless, ramped up their rhetoric on Monday.

Numan Kurtulmus, a deputy prime minister, predicted that the Netherlands would have to apologize to Turkey for blocking Cavusoglu from flying to the country, and for expelling the family affairs minister, Fatma Betul Sayan Kaya, after she had entered by land from Germany. "You will see that, in the end, they will come to the point where they will apologize," Kurtulmus said, without elaborating, according to the Turkish state-run Anadolu News Agency.

He said the treatment of the ministers echoed "footsteps of the far-right, of the neo-fascism and neo-Nazism that has been on the rise in Europe in the past five or six years."

The Turkish Foreign Ministry also said it had summoned the Netherlands' top diplomat in Ankara, the capital, as a formal protest against the treatment of the family affairs minister and what it called a "disproportionate" use of force against protesters at a rally in Rotterdam that ended on Saturday with several arrests after clashes with police.