About 400 migrants are feared drowned after their boat capsized off Libya, survivors have told Save the Children.
The Italian coast guard had rescued 144 people on Monday (local time) and launched an air and sea search operation in hopes of rescuing others.
Save the Children said that many of the survivors were "young men, probably minors".
The European Union says that more than 7,000 migrants have been rescued from the Mediterranean since Friday.
Survivors say that the latest tragedy happened after the boat, carrying about 550 migrants in total, overturned a day after leaving Libya.
Nine bodies have already been recovered, but no more survivors have been found since then.
Michele Prosperi from Save the Children Italy told the BBC it was still trying to find out how many young people were on the boat. "There were a few minors" among the survivors, he said.
The migration season had just started, and "when the weather improves... we may have many more arrivals", he added.
Late last year, the EU scaled back its maritime rescue operations, as some argued the service could encourage more migrants.