The Italian navy said Sunday it has arrested 16 human traffickers aboard a so-called "mother ship" in international waters off Libya.
Submarine surveillance ahead of Saturday's raid was "of particular importance, notably for gathering proof" against the traffickers, it said in a statement.
The fishing boat served as a staging area from which the traffickers dispatched their clients -- Syrian migrants seeking asylum or a better life -- aboard smaller, often rickety vessels to destinations in southern Europe.
The boat had been under surveillance, including by drones, hi-tech radars, night-vision equipment and the submarine, under stepped-up operations launched after two shipwrecks last month claimed some 500 lives.
It was located some 500 kilometres (300 miles) southeast of Sicily, near Libya, the navy said.
Also Saturday, the navy rescued 176 Syrian migrants from one of the boats dispatched by the "mother ship", it said.
The navy said the "mother ship" itself had sunk after it took on water while being towed towards an Italian port.