U.S. Defense Secretary Ash Carter says NATO military authorities have been ordered to draw up plans for how the alliance could help shut down illegal migration and people smuggling across the Aegean Sea.

Carter made the announcement at a Thursday press conference after three NATO allies — Turkey, Germany and Greece — requested alliance participation in an international effort to help end Europe's gravest migration crisis since World War II.

The three countries made the request at a NATO defense ministers' meeting in Brussels. NATO ministers "tasked NATO military authorities to provide its advice for options for implementing it," said Carter.

Carter says those recommendations will be reviewed by NATO's Military Committee and submitted to the North Atlantic Council, NATO's chief decision-making body.