Nato ships are being deployed to the Aegean sea to deter people-smugglers taking migrants from Turkey to Greece.
The announcement followed a request from Turkey, Germany and Greece at a defence ministers’ meeting in Brussels.
Nato chief Jens Stoltenberg said the mission would not be about “stopping or pushing back refugee boats”.
Nato, he said, will contribute “critical information and surveillance to help counter human trafficking and criminal networks”.
The decision marks the security alliance’s first intervention in Europe’s migrant crisis.
Nato’s top commander, General Philip Breedlove, said the warships would arrive within 24 hours and that he was putting together a mission plan.
US defence secretary Ashton Carter earlier said that targeting the “criminal syndicate that is exploiting these poor people” would have the greatest humanitarian impact.
The decision was made to help Turkey and Greece “manage a human tragedy in a better way than we have managed to do so far”, Mr Stoltenberg, Nato Secretary General, said.