Distress calls were made from 17 boats, and several other rescue operations were still underway late yesterday.
The 17 bodies were found on three inflatable dinghies, from which more than 300 other migrants were rescued alive, the Italian navy said on Twitter.
The navy's press office, contacted by AFP, was not immediately able to say how the migrants died.
But the Italian authorities have in the past spoken of the harsh conditions faced by the migrants at sea, where they have to endure extreme weather changes and are at risk of hunger, thirst and violence on board the often crammed and flimsy vessels.
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A similar international maritime rescue mission on Thursday saw more than 700 migrants helped to safety off the coast of Sicily after they had set sail from Libya in six boats.
So far this year, some 1,770 migrants have perished on the hazardous journey to Europe, according to the International Organisation for Migration (IOM), a 30-fold increase on the same period in 2014.
The huge spike in the number of people trying to cross the Mediterranean Sea in recent weeks has been attributed to the worsening security situation in Libya - the staging post for most of the crossings - as well as milder weather.
"It happens a lot in waves, you could have a few days where nothing happens, then there can be a high number of arrivals at the same time," Flavio di Giacomo, a spokesman for the IOM in Italy, told AFP.
After a string of deadly shipwrecks that sparked global alarm, EU ministers this month approved plans for a military operation to fight people smugglers in the Med, although proposals to destroy traffickers' boats in Libyan waters still need UN approval.