About 500 migrants are feared dead after their ship was rammed by another boat near Malta last week, a migration body said.
The International Organisation for Migration said it spoke to two survivors rescued on Thursday.
They said the boat, which had been carrying women and children, had been intentionally sunk by traffickers.
News of the sinking near Malta emerged as another vessel carrying 250 people sank off the coast of Libya.
Over 200 people are feared to have drowned in that incident.
Thousands of migrants have drowned in recent years as they attempt to reach Europe from North Africa and the Middle East.
Many do so in unsafe and overcrowded vessels.