ROME (AFP) - Survivors of a fishing boat shipwreck in the Mediterranean off the coast of Libya on Wednesday said that some 100 migrants may have drowned, the International Organisation of Migration (IOM) said.

"The survivors who talked to our teams when they landed at Porto Empedocle (Sicily) spoke of 100 missing people who were stuck in the hull," IOM spokesman in Italy Flavio Di Giacomo told AFP on Thursday.

According to those who made it to land, the boat was carrying some 650 people when it set sail from chaos-wracked Libya, "most of them Moroccans", Di Giacomo said.

Earlier estimates had put the toll of Wednesday's disaster at five.

While the Italian navy was able to rescue 562 people from the water, Wednesday's shipwreck may be one of the worst tragedies since the migrant crisis began.

Di Giacomo said it was rare for such large, heavy ships to be used by migrants seeking to reach Europe from Libya.

As well as Moroccans, many Tunisians and two Syrian families who had been living in Libya were among the survivors, he added.