About 400 migrants are feared drowned after their boat capsized off Libya, survivors have told Save the Children.
The Italian coast guard rescued 144 people from the boat on Monday and launched an air and sea search operation in hopes of saving others.
Hundreds more migrants rescued from boats in the Mediterranean are due to arrive in Sicily during the day.
More than 8,000 migrants have been picked up since Friday, and more boats are heading for the Italian coast.
Italy's interior ministry has instructed officials throughout the country to be prepared to house the new arrivals, many of whom are children.
UN officials say well over 500 people have died since the start of the year, 30 times more than in the same period last year.
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.
Save the Children said that many of the survivors were "young men, probably minors".
Save the Children Italy was still trying to find out how many young people were on the boat, spokesman Michele Prosperi told the BBC. "A few minors" were among the survivors, he said.
The migration season had just started, and "when the weather improves... we may have many more arrivals", he added.
Last year, 170,000 migrants crossed the Mediterranean to Italy and as many as 3,500 died while making the journey, officials say.
The Italian government's maritime rescue operation was scaled back, amid concerns that it was encouraging migrant crossings, and a more limited EU border security operation took over.
However, the latest numbers show that the EU's policy of deterring people is not working, the BBC's James Reynolds in Sicily reports.
UN High Commissioner for Refugees spokesman Laurens Jolles told the BBC the capacity to rescue was not as strong as it had to be and a far greater response was needed.
Meanwhile the EU's Frontex border agency said that people smugglers had fired shots into the air to warn away another coast guard vessel rescuing migrants.
The incident on Monday happened about 60 nautical miles off Libya after an Italian vessel and an Icelandic coast guard ship came to the rescue of 250 migrants on a tugboat.
After most of the migrants were rescued, traffickers in a speedboat drove towards the rescuers, firing shots before retrieving the now-empty migrant 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.
EU migration commissioner Dimitris Avramopoulos told the European Parliament: "The unprecedented influx of migrants at our borders, and in particular refugees, is unfortunately the new norm and we will need to adjust our responses accordingly."
Over 280,000 people entered the EU illegally last year, many fleeing conflict in Syria and repression in Eritrea.
Since the fall of Muammar Gaddafi in 2011, Libya has been without a stable government allowing trafficking networks to thrive.
On Friday, humanitarian organisation Medecins Sans Frontieres announced it would operate a search and rescue mission in the Mediterranean between May and October.
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