CAIRO — As they scrambled to curb the flow of migrants, Europe’s leaders wrestled with a vexing question: How to stop the ruthless Libyan militias that control the human-trafficking trade from dispatching countless boats across the Mediterranean?

Now Italy, after striking out on its own, appears to have found a solution — one that, though wildly successful for the moment, is provoking questions about its methods and the humanitarian costs.

Arrivals of migrants in Italy have plunged in recent months. In August alone, they fell 85 percent, leading some to charge that Italy was paying off Libya’s most rapacious warlords at the risk of further destabilizing the fractured North African country, while condemning migrants to misery.

Human rights activists liken the grimy conditions at militant-run detention centers inside Libya to concentration camps, while the top United Nations human rights official, Zeid Ra’ad al-Hussein, recently warned that the Italian-led tactics were “very thin on the protection of the human rights of migrants inside Libya and on the boats.”