The vessel set off with 86 people on board, the International Organization for Migration in Libya said, and women and children were among those who died in the shipwreck.
Survivors said that 41 people, shunted by traffickers onto a small makeshift iron vessel, had drowned after it capsized en route from Tunisia to Italy.
Greek authorities blamed smugglers for a disaster that may have been one of the worst of its kind. Critics say tougher policies are increasing the risks.