Some 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 and immediately 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 plucked from the Mediterranean since Friday.
Survivors say that the latest tragedy happened after the boat, carrying about 550 migrants in total, flipped a day after after leaving the Libyan coast.
Nine bodies have already been recovered, but no more survivors have been found since the search was launched.
The UN refugee agency said that the suggested death toll was likely to rise given the size of the ship.
No more survivors have been found since the search began
Meanwhile the EU's Frontex border agency said that people smugglers had fired shots into the air to warn away a coast guard vessel attempting to rescue migrants.
The incident on Monday happened some 60 nautical miles off Libya after an Italian coast guard ship came to the rescue of 250 migrants as traffickers attempted to retrieve a boat.
The coast guard vessel was already carrying 342 migrants from a previous rescue.
Frontex says the incident shows that traffickers are running out of boats.
More than 280,000 people entered the EU illegally last year, many fleeing from conflicts in from Syria, Eritrea and Somalia.