With the exchanges worsening Turkey's already fraught relations with EU, Brussels sternly warned Ankara to avoid intensifying the crisis.

President Recep Tayyip Erdogan on Monday accused German Chancellor Angela Merkel of backing terrorists, as new acrimony erupted between Turkey and its EU allies.

Mr. Erdogan's scathing broadside came hours after the European Union urged him to avoid inflammatory rhetoric in a growing standoff with Germany and the Netherlands over the blocking of ministers seeking to address rallies promoting a 'yes' vote in a April 16 referendum on giving him greater powers.

Mr. Erdogan had twice over the weekend accused NATO ally The Netherlands of acting like the Nazis, comments that sparked outrage in a country bombed and occupied by German forces in World War II.

With the exchanges worsening Turkey's already fraught relations with the European Union, Brussels sternly warned Ankara to avoid intensifying the crisis.

EU foreign policy chief Federica Mogherini and EU Enlargement Commissioner Johannes Hahn called on Turkey to "refrain from excessive statements and actions that risk further exacerbating the situation."

But in an interview with A-Haber television, Mr. Erdogan bluntly stated: “Mrs Merkel, you are supporting terrorists.”

He accused Berlin of not responding to 4,500 dossiers sent by Ankara on terror suspects, including those linked to Kurdish militants and the failed coup in Turkey last year.

“Mrs Merkel, why are you hiding terrorists in your country?... Why are you not doing anything?”

Ms. Merkel's spokesman described the accusations as “absurd”, saying the Chancellor had no intention of taking part in a “competition of provocations”.

'Completely wrong comparison'

Turkey's EU Affairs Minister Omer Celik also said Ankara "should re-evaluate" a key part of a 2016 deal to stem the flow of migrants to the EU.

He said Turkey should look at its policy on preventing migrant flows across land borders, although it would keep halting the illegal and dangerous sea crossings as a matter of human responsibility, state media said.

Dutch authorities at the weekend prevented the plane of Turkish Foreign Minister Mevlut Cavusoglu from landing and blocked Family Minister Fatma Betul Sayan Kaya from holding a rally in Rotterdam.

The ministers had been seeking to harness the support of an estimated 400,000 people of Turkish origin living in the country ahead of the April 16 referendum on constitutional changes.

Bringing out the millions-strong expatriate vote could be key in a referendum that could be a turning point in Turkey's modern history.

Mr. Erdogan said the ministers would make the “necessary applications” to the European Court of Human Rights over their treatment

Meanwhile Dutch Prime Minister Mark Rutte is under pressure to take a hard line against Erdogan as he faces the far-right populist Geert Wilders in general elections Wednesday.

The Turkish foreign ministry on Monday summoned the Dutch envoy to Ankara for the third day in a row.

The Netherlands also issued a new travel warning to Dutch citizens, urging them to stay “alert across the whole of Turkey”. And Germany warned its citizens to take note of "increased political tensions and protests that could be directed against Germany".

Turkey had already responded furiously to fellow NATO ally Germany's refusal to give permission for ministers to hold rallies there, with Erdogan comparing such action to “Nazi practices”.

After Erdogan used the same language to scold the Netherlands, Ms. Merkel said on Monday that the comparison was "completely wrong" and “banalises suffering”.

'Lifeline for Erdogan'

The issue risks spiralling into a crisis with the EU as a whole, which Turkey has sought to join for more than half a century.

Mr. Erdogan, who has indicated he may personally travel to EU states to address rallies said Sunday that the West was showing its "true face" in the standoff.

Deputy Prime Minister Nurettin Canikli described Europe as a "very sick man", a nod to the moniker used in the mid-19th century of the declining Ottoman Empire as the "sick man of Europe".

In a new onslaught against Rutte, Erdogan told A-Haber television that the Dutch leaders "haven't understood democracy."

The immediate effect of the row, however, may aid Mr. Erdogan, Soner Cagaptay, director of the Turkish Research Programme at The Washington Institute, told AFP.

“By blocking (the rallies), they may have given Erdogan a lifeline to eke out a victory in the referendum,” he said.