Nato ships are being deployed to the Aegean sea to deter people-smugglers taking migrants from Turkey to Greece, Nato chief Jens Stoltenberg says.
The announcement followed a request from Turkey, Germany and Greece at a defence ministers' meeting in Brussels.
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".
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," Stoltenberg, Nato Secretary General, said.
Nato's Standing Maritime Group 2, which is under German command, will lead the operation in co-operation with Greek and Turkish coastguards.
Stoltenberg said reconnaissance and intelligence gathering was also being stepped up at the Turkey-Syria border.
Almost 75,000 migrants and refugees have already arrived in Greece by sea in 2016, the UN refugee agency says.
Separately, two people-smugglers are on trial in Turkey in connection with the death of Alan Kurdi and four other people.
Alan was the Syrian three-year-old whose photograph - lying dead on a Greek beach - focused world attention on the refugee crisis.
The International Organization for Migration recently said that 409 people had died so far this year while attempting to cross the Mediterranean Sea.