Up to 400 illegal migrants trying to reach Europe died after their boat capsized off the Libyan coast on Sunday, said survivors who were saved and brought to Italy.
The Italian coastguard had previously said that they managed to rescue 144 of the people on the capsized vessel, while nine bodies were also recovered.
Quoting survivors, the International Organisation for Migration and the charity Save the Children said between 144 and 150 survivors arrived on Tuesday morning at Reggio Calabria, located on Italy's southern tip.
"There were 400 victims in this shipwreck, which occurred 24 hours after (their vessel) left the Libyan coast," Save the Children said in a statement, citing survivors.
"There were several young males, probably minors, among the victims," the international NGO said, adding that there were also children among the survivors.
IOM spokesman in Italy Flavio Di Giacomo told AFP his organisation has also interviewed several survivors, who said there were between 500 and 550 people on board when the ship sank.
"We are continuing to investigate in order to understand how the shipwreck happened," Di Giacomo said.
According to initial findings, the boat may have capsized after passengers started moving when they saw the Italian rescue team.