More than a dozen migrants drowned off the coast of Turkey on Sunday, adding weight to a UN warning of an unfolding humanitarian emergency along the Balkan migration route and upping the stakes for an EU-Turkey summit on the refugee crisis.

Eighteen migrants drowned in the Aegean Sea off the coast of Turkey's western district of Didim after their boat got into trouble, Turkey's official Anadolu news agency reported.

Fifteen migrants were rescued, according to the report. The migrants' nationalities were not mentioned. Separately, Greek and EU officials said that about 400 migrants had been rescued from the Aegean in the last 24 hours.

The new deaths follow warnings from a UN refugee agency representative that a humanitarian crisis is unfolding among refugees massed at Greece's border with Macedonia. Many of those migrants who make it to Turkey then head to Greece in hopes of pushing further into Europe.

The governor of Greece's Central Macedonia province said at least 13,000 people hoping to claim asylum in Western Europe are stuck at Greece's Idomeni border crossing. With Macedonia only allowing around 250 people per day to pass, the situation is rapidly deteriorating as more people arrive daily, according to observers.

"It's a wake-up call for the EU leaders, because this is a humanitarian crisis," warned UNHCR's Babar Baloch from the Idomeni refugee centre.

Baloch said about 55 per cent of the people at the camp - which was designed for 2,000 people - are women and children. Many people have taken to camping in the open in nearby fields. Many of the children are reportedly suffering from respiratory diseases.

The European Union has planned a crisis summit with Turkey on Monday to discuss the refugee crisis. Many asylum seekers fleeing conflict in the Middle East transit through the country before setting off for Europe.

Turkish Prime Minister Ahmet Davutoglu will attend the summit in Brussels. He was due to hold preparatory talks with German Chancellor Angela Merkel in the Belgian capital on Sunday evening.

EU officials have said that finding a way to keep the people from leaving Turkey in the first place is a critical part of the plan to control the number of people streaming into Europe.

NATO has launched an operation in the Aegean Sea to monitor migrant smuggling between Turkey and Greece. Hopes are high that its presence will help deter migrant crossings.

The military alliance announced Sunday that it will now start operating in Greek and Turkish waters, after consultations with the two NATO members.

"The purpose of NATO's deployment is not to stop or push back migrant boats, but to help our allies Greece and Turkey, as well as the EU, in their efforts to tackle human trafficking and the criminal networks that are fuelling this crisis," NATO Secretary General Jens Stoltenberg said in a statement.

"The decision of NATO to assist in the conduct of reconnaissance, monitoring and surveillance of illegal crossings in the Aegean Sea is an important contribution to international efforts to tackle smuggling and irregular migration," EU foreign policy chief Federica Mogherini and EU Migration Commissioner Dimitris Avramopoulos said.

Germany has been one of the top destinations for refugees, after Merkel suspended EU migration regulations in August last year and allowed Syrian refugees to register even if they had entered the EU elsewhere, often in Greece.

But on Sunday, Merkel's vice chancellor ruled out a repeat of last summer's open-door policy, saying Germany will not simply take in refugees who are now stuck at border crossings around Europe.

"Germany cannot repeat this gesture, because as it turned out, we remained mostly the only ones in Europe to make it," Vice Chancellor Sigmar Gabriel says in comments released in advance of Monday's edition of widely read German tabloid Bild.

Gabriel also criticized Germans who are clandestinely happy about Balkan states' decisions to manage the migration crisis on their own with border restrictions and closures.

"Some in Germany secretly delight in the fact that the Balkan countries are curbing the influx with border closings and, in a way, doing the dirty work for Germany," Gabriel said in the Bild report.

"In the long run, that doesn't help," he added. "The refugees are seeking other paths for themselves. In the end, the Mediterranean will again become a mass gravesite."

The Greek Foreign Ministry has threatened Macedonia with unspecified serious consequences if it does not let the refugees pass through.

"Greece needs help to confront this situation," warned Baloch. "Desperate refugees will suffer more."

The Greek government announced on Sunday that a health centre for refugees at Idomeni should be operational by Wednesday.

Governor Apostolos Tzitzikostas has called on the government to declare a state of emergency in the region.

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