NATO is sending military vessels to the Aegean Sea to help Turkey and Greece tackle criminal networks that smuggle refugees into Europe, the 28-nation military alliance has said.

Jens Stoltenberg, NATO's secretary-general, said on Thursday, after the meeting of the bloc's defence ministers in Brussels, that the mission would focus on gathering intelligence, but did not give many details.

"This is not about stopping and pushing back [refugee boats] ... but about critical surveillance to help counter human trafficking and criminal networks," he said.

Turkey, Germany agree on plan to ease refugee crisis

Stoltenberg said that the refugee crisis, driven largely by conflict and turmoil in Syria, posed a major security threat to the alliance.

Germany said it would take part in the NATO mission along with Greece and Turkey, while the United States, NATO's most powerful member, said it fully supports the plan.

"There is now a criminal syndicate that is exploiting these poor people and this is an organised smuggling operation," said Ash Carter, US secretary of defense.

"Targeting that is the way that the greatest effect can be had ... That is the principal intent of this," Carter said.

Denmark is also expected to contribute a ship.

Intelligence gathered about people-smugglers is likely to be handed over to Turkish coastguards to allow them to combat the traffickers more effectively, rather than having the NATO alliance act directly against the criminals, NATO diplomats said.

Greek-Turkish issue

"German, Turkish, Greek and Canadian ships are already there," said Al Jazeera's Paul Brennan, reporting from Brussels.

"The thing that has held up problems so far is the rivalry between Greece and Turkey. Neither wants the ships of the other's navy in their territorial waters."

Greek and Turkish ships will remain in their respective territorial waters, given sensitivities between the two countries, and any refugees saved by NATO vessels will be returned to Turkey.

The European Commission welcomed the plan, but said it aimed to create its own border and coastguard system to fulfil the same function.

Syria's War: Refugees flee to Turkey for safety

"This is a decision we welcome. We hope that lives will be saved in the Aegean Sea," commission chief Jean-Claude Juncker's spokesman Margaritis Schinas said.

"We regard this as very much a sort of a forerunner of the European coastguard, going back to proposals made in December." 

Turkish transit

Turkey was the main transit country for the more than one million people who reached Europe last year.

Having reached Greece, most of them made their way north to Germany and other richer countries in the European Union.

More than 70,000 made the dangerous crossing in January, with more than 400 dying, according to the International Organization for Migration.

Hundreds of thousands more are expected to follow this year, with no prospect of a negotiated solution to the war in Syria, which has killed more than 260,000 people and displaced half the population since March 2011.

Source: Al Jazeera and agencies