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 Organization 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.
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.
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"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.
On Tuesday, the coastguard said its teams had rescued nearly 8,000 people at sea since Friday.
An increasingly violent and chaotic situation in Libya, a key jumping off point for migrants, has also helped prompt the huge hike in the number of asylum seekers trying to make it to Europe.
According to Italian authorities more than 15,000 migrants have arrived in Italy so far in 2015. There were 15,000 in April alone last year and an average of 25,000 each month between June and September.
People smuggling remains a lucrative business for traffickers, with refugees and migrants rescued in the Mediterranean in February saying they paid between USD 500-1,000 for their crossing in boats that are often no more than rubber dinghies.