At least 70 migrants are missing after their inflatable dinghy sank just off the Libyan coast, according to 26 survivors rescued by an Italian ship.
Italian media gave the figure of 70 while the International Organization for Migration (IOM) said it believed 84 were missing.
The dinghy was found taking on water in rough seas after the Italian coast guard received a satellite phone call.
It diverted the merchant ship to rescue survivors, who were brought to Italy.
A coast guard spokesman said similar boats used by people smugglers could hold 100-120 people, and were usually full.
No details of the nationalities of the migrants, who were brought to the island of Lampedusa, were immediately available.
Around 27,000 refugees and other migrants have reached Italy by boat so far this year, most of them setting out from Libya.
Officials expect the number attempting the illegal crossing to rise because countries in the Balkans have shut their land borders.
At least 800 migrants are feared to have drowned in the southern Mediterranean this year to date.
A note on terminology: The BBC uses the term migrant to refer to all people on the move who have yet to complete the legal process of claiming asylum. This group includes people fleeing war-torn countries such as Syria, who are likely to be granted refugee status, as well as people who are seeking jobs and better lives, who governments are likely to rule are economic migrants.