HAMBURG, Germany — The Panther, a German-owned merchant ship, is not in the business of sea rescues. But one day a few months ago the Libyan Coast Guard ordered it to divert course, rescue 68 migrants in distress in the Mediterranean and return them to Libya, which is embroiled in civil war.

The request, which the Panther was required to honor, was at least the third time that day, Jan. 11, that the Libyans had called on a merchant ship to assist migrants.

The Libyans could easily have alerted a nearby rescue ship run by a Spanish charity. The reason they did not goes to the core of how the European authorities have found a new way to thwart desperate African migrants trying to reach their shores from across the Mediterranean.

And some maritime lawyers think the new tactic is unlawful.

Commercial ships like the Panther must follow instructions from official forces, like the Libyan Coast Guard, which works in close cooperation with its Italian counterpart.