Around 400 Libyan migrants are feared drowned after their boat capsized en-route to Italy, according to the charity, 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, reported the BBC on Wednesday.
Survivors said that the 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".
At the weekend Italian rescue services saved 5,629 migrants and refugees sailing from Libya to Italy during the last few days, officials have reported.
Hundreds more migrants rescued from boats in the Mediterranean are due to arrive in Sicily during the day.
Aid agencies are predicting a slew of migration across the Mediterranean this summer as a result of continued and worsening chaos in North Africa and the Middle East.
Italy's interior ministry has instructed officials throughout the country to be prepared to house the new arrivals, many of whom are children.
UN High Commissioner for Refugees spokesman Laurens Jolles said the capacity to rescue was not as strong as it had to be and a far greater response was needed.
The migrants, including Africans fleeing extremism, poverty and oppression, and Syrians escaping their country's horrific civil war, were packed into 20 rickety fishing boats and overloaded inflatable dinghies, according to officials.