About 400 migrants are reported to have died at the weekend when their boat capsized on the way from Libya to Italy.
The Save the Children charity cited survivors who were rescued from another boat and brought to an Italian port.
The charity said "many young men, probably minors" were among the victims.
It is the latest tragedy in the Mediterranean where the death toll from shipwrecks has surged this year.
More than 500 people have been killed trying to cross the Mediterranean from Africa, which is up sharply from 47 in the same period of 2014.
The number of boats carrying migrants aiming to reach the EU from Africa has picked up in recent weeks as fine spring weather makes the passage safer.
In February, more than 300 drowned when attempting the crossing in cold weather and rough seas.
Save the Children and other humanitarian organisations have called for the European Union to bolster its sea rescue operations before the migrant flows soar as they usually do in the summer months.
Yesterday, 2,851 migrants were saved in rescue operations in the Mediterranean, the Italian coastguard said, adding to at least nine who died and 5,629 who were saved over the weekend.
