Several survivors have reported that the vessel carried 650 people when it sank on Friday en route from Egypt to Italy, the aid and advocacy group said in Geneva.
In addition to nearly a dozen corpses that were recovered last week,320 people remain missing and are presumed to have drowned, spokesman Joel Millman reported.
One 3-year-old girl survived because her father entrusted her to another migrant, asking him to look for her relatives in Egypt.
The girl's father, mother and two brothers died, the migrant told IOM staff after he was rescued along with other survivors.
Including this latest accident, 2,809 have perished in the Mediterranean this year as they tried to reach Southern Europe in search of refugee protection or a better livelihood.
"Fatalities on the Mediterranean Sea in 2016 are nearly 1,000 ahead of last year's mid-year total," Millman said.
He said there was a sharp rise in children trying to cross the sea to join family members who are already in Europe.
"Visa processes take a long time, so there is a strong urge to give your children into the hands of smugglers," he said.
The biggest national groups arriving in Italy are from Eritrea, Nigeria and Gambia, while those who have reached Greece this year were mainly from Syria, Afghanistan or Iraq.
- DPA