Senior EU and NATO officials pleaded for restraint Monday as a nasty political feud between Ankara and European capitals over Turkey's upcoming constitutional referendum continued to escalate.

All sides were seething after a weekend in which the Netherlands barred Turkish Foreign Minister Mevlüt Çavuşoğlu from entering the country to attend a rally in Rotterdam in support of President Recep Tayyip Erdoğan's constitutional changes, which European experts have said could potentially undermine the country's rule of law. Erdoğan, in turn, responded by comparing Dutch leaders to Nazis, echoing a comment he made about the German government last week.

The furor seemed to push the already tense relations between Europe and Ankara to the brink, raising concerns that a fragile deal to stem the flow of migrants to Europe might fall apart. And it prompted senior leaders, including NATO Secretary-General Jens Stoltenberg, to call on Turkey and its European allies to focus on the more important goals they share including crucial military cooperation in the fight against Islamic State, rather than on a political spat.

"Robust debate is at the heart of our democracy but so is also mutual respect," Stoltenberg said at a news conference Monday. "Therefore, I will encourage all allies to show mutual respect, to be calm and have a measured approach ... to defuse tensions and de-escalate the situation."

"It's important that we now focus on everything that unites us," Stoltenberg added. "NATO presence in Turkey is good for Turkey but it is also good for Europe and the rest of the alliance."

In a sign of just how tense the crisis had become, there was talk in the hallways of NATO headquarters Monday about the Suez Canal crisis in 1956.

But far from stepping back, the Turkish government announced late Monday that it would retaliate against the Netherlands with diplomatic sanctions: barring the return of the Dutch ambassador who is currently out of Turkey; closing Turkish airspace to official Dutch flights; suspending inter-governmental meetings, and dissolving a friendship group between the Turkish National Assembly and the Dutch Parliament. A senior government spokesman, Numan Kurtulmuş, who announced the sanctions, repeated the assertion of “neo-Nazi influence” in the Dutch government and said Ankara was acting to protect its “rights and dignity.”

“Turkey will act responsibly and decisively,” Kurtulmuş said.

In a sign of just how tense the crisis had become, there was talk in the hallways of NATO headquarters Monday about the Suez Canal crisis in 1956, when Britain and France bombed Egyptian positions without alerting other allies including the U.S. The point was that things could be worse, but it offered little reassurance, given that relations with Turkey have been in a sharp downward spiral since the failed coup against Erdoğan's government last summer.

The Turkish president responded with a fierce crackdown that included jailing hundreds of military officers, journalists and civil servants, including members of the judiciary. Those steps, which Ankara viewed as crucial to its security and Europe viewed as overly authoritarian, had already led Members of the European Parliament to call for an end to accession talks with Turkey, which is formally still a candidate for EU membership.

The current feud stems from efforts by Erdoğan's government to build support among Turks living abroad for the package of constitutional changes with campaign-style events. Austria, Germany and the Netherlands each took steps to prevent such campaigning, raising questions about the potential suppression of free speech that were quickly answered with concerns about public safety and the technical requirements of obtaining permits for rallies and demonstrations.

At the European Commission on Monday, officials sided firmly with the Netherlands, Germany and other members of the bloc.

"The European Union calls on Turkey to refrain from excessive statements and actions that risk further exacerbating the situation," the Commission's chief spokesman, Margaritis Schinas, said at a news briefing.

Schinas noted that Commission President Jean-Claude Juncker and the EU's foreign policy chief, Federica Mogherini, had worked through the weekend monitoring the tensions with Turkey.

"Matters of concern can only be resolved through open and direct communication channels and we, here at the Commission, will continue to provide our good offices in the interest of EU-Turkey relations," Schinas said, but he pointedly noted: "Decisions with regard to the holding of meetings and rallies in our member states are a matter for the member states concerned in accordance with the applicable provisions of international and national law."

Rutte's tough response, blocking the Turkish foreign minister's plane from landing Saturday and forcibly ejecting another Turkish minister who drove to Rotterdam from Germany, seem to have bolstered his position.

At a news conference Monday, Rutte said that he had tried to reduce tensions but that Turkey was unwilling to go along, and he insisted that his position had not been altered by the imminent elections.

“We tried to deescalate the conflict,” Rutte said, adding, “To deescalate, it takes two to tango.”

"It is of concern that this process of constitutional change is taking place under the state of emergency" — Federica Mogherini

The Dutch prime minister also said that he was pleased by the show of European solidarity. “I was happy with [German Chancellor Angela] Merkel, she told us that Germany is shoulder to shoulder with us,” he said, “I think the French ministers were helpful too. And the European Commission, all supporting our position, that is very positive.”

With each side seeming to benefit politically from the dispute, the chances of tempers cooling anytime soon appeared slim.

In a joint statement, Mogherini and Commissioner Johannes Hahn, who is responsible for neighborhood policy and EU enlargement issues, warned sharply about Erdoğan's proposed constitutional changes, citing the Venice Commission's warning of a rollback of democratic freedoms in favor of a heavy-handed presidency.

"The Venice Commission's comments on the proposed constitutional amendments raise serious concerns at the excessive concentration of powers in one office, with serious effect on the necessary checks and balances and on the independence of the judiciary," Mogherini and Hahn said. "It is also of concern that this process of constitutional change is taking place under the state of emergency."

They also warned that approval of the constitutional changes could further undermine Turkey's prospects of ever joining the EU. "The proposed amendments, if approved at the referendum of 16 April, and especially their practical implementation, will be assessed in light of Turkey's obligations as an EU candidate country and as a member of the Council of Europe," they said.

In a statement Sunday, the Dutch government said it had canceled a scheduled rally in Rotterdam, and subsequently blocked the Turkish foreign minister’s plane from landing because of public safety concerns, calling Turkey’s rhetoric “unacceptable.”

The Dutch foreign ministry also issued a travel warning to its citizens noting the “diplomatic tensions between Turkey and the Netherlands” and adding to pre-existing travel warnings about the danger of areas of Turkey near the Syrian border. “Be alert across Turkey, avoiding gatherings and crowded places,” the new warning stated. “Follow the news of the embassy and the consulate general.”

Cynthia Kroet contributed to this article.

This article has been updated with the most recent developments from Turkey.