NATO vessels have arrived in the Aegean Sea to start monitoring the migration route between Turkey and Greece, a NATO official told dpa on Friday, as the military alliance prepares to help gather information on smuggling networks.

The short sea passage has been the main crossing point for migrants and asylum seekers heading to Europe, but has claimed several hundred lives this year already.

Europe has been struggling to deal with a surge in migration that saw more than 1 million people arrive in 2015. Many are fleeing the war in Syria, but economic migrants have also joined their ranks.

NATO defence ministers agreed last week to deploy ships as part of an effort to stem illegal migration flows, following a request from Turkey, Greece and Germany.

Four vessels under German command, including personnel from Canada, Greece and Turkey, have arrived in Aegean international waters and are conducting "routine reconnaissance, monitoring and surveillance activities," a NATO official said Friday. The ships were rerouted from another part of the Mediterranean Sea.

The ultimate aim is to feed information to the national authorities in Greece and Turkey, in cooperation with the European Union's border agency FRONTEX, focusing on human trafficking and criminal networks fuelling the migration crisis.

The full details of the mission are still being finalized.

"This is not about stopping or pushing back refugee boats," NATO Secretary General Jens Stoltenberg said last week.

But any migrants in distress who are rescued by NATO vessels will be returned to Turkey, sending a "clear signal to the smuggling gangs," German Defence Minister Ursula von der Leyen has said.

EU leaders meeting in Brussels have also been discussing efforts to curb the migration flows, urging member states and partner country Turkey to implement jointly agreed measures.

German Chancellor Angela Merkel met with Greek Prime Minister Alexis Tsipras and French President Francois Hollande on Friday, after a migration debate with all 28 leaders late Thursday that an EU source said proved "more difficult than expected."

Tsipras reportedly caused irritation in Brussels by seeking guarantees on the migration crisis as a condition for his endorsement of a reform deal for Britain, which is also being negotiated at the summit on Friday.

Greece has come under pressure from the EU to ratchet up the control of its sea frontiers, as nations further along the migration route to northern Europe are closing up their borders - prompting fears in Athens that thousands of migrants will be stranded in its country.

Hollande lent his support to European plans to establishing new registration centres in Greece and deploying a coastguard during an interview with France Inter radio from Brussels on Friday evening.

Closing internal borders in Europe would abandon Greece at the frontlines of the influx of people seeking refuge, he said.

"We cannot place on one country a responsibility that is shared by all," Hollande said, adding that retaining the Schengen system of open borders within the bloc was vital for Europe.

Tsipras had demanded in his meeting with Hollande and Merkel that the other EU member states refrain from unilateral border closures at least until March 6, government sources in Athens said on condition of anonymity.

Hollande and Merkel assured Greece that it would get "every support that it needs," the sources said.

Tensions have been rising in the EU since Austria decided to limit the number of asylum seekers it would let in, setting off fears that other countries upstream on the so-called Balkan migration route will become backed up.

Balkan nations are already following Austria's cue for heightened border security, with plans to stop migrants at their own frontiers.

Hungary will seal the three railway crossings it shares with Croatia starting February 21, "in the interest of public security," media reports said Friday. The measure will be in effect for 30 days.

Hungary was on the path of migrants arriving through Serbia and Croatia until it sealed its borders with the two countries in September and October.

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