The report is one of many describing migrants dying at sea. Thousands of people are leaving North Africa every month aboard crowded vessels to escape desperate conditions and reach European shores.
Sunday's deaths happened as a rescue vessel approached the migrants, Save the Children said, citing interviews it conducted with survivors after they arrived at the port of Catania, on the Italian island of Sicily. The aid organization said it feared that 40 people had died.
"The commercial ship Zeran picked up two separate dinghies since Sunday, both of which set off from Libya," Save the Children said in a statement.
"One had more than 100 people, all survivors," the statement said. "The other had approximately 137 people on board. They have all been brought to Catania port in Sicily this morning and are disembarking right now. They are from Ghana, Gambia, Senegal and Ivory Coast (so far). We believe there are at least 30 minors (under 18s) so far but all the arrivals still need to be interviewed. There are at least 2 younger children (5-7 years old)."
Dozens reportedly fell into the sea
The statement said that Save the Children's Giovanna di Benedetto had interviewed some of the men from the second dinghy.
Three of those men reported that "dozens of people had fallen into the sea when they saw the commercial boat approach and they had drowned as they couldn't swim. Five dead bodies have been brought to Catania," Save the Children said.
The aid organization said it was not yet clear whether the bodies were those of people who drowned or whether they were already dead on the dinghy.
Italian coast guard recovers 10 bodies
The Italian coast guard said it couldn't confirm the charity's report, though it said 10 bodies were recovered Sunday in three separate rescue operations in the Mediterranean. It said six operations were currently going on.
The coast guard said that, between Friday and Tuesday, 8,300 migrants had been saved.
As of mid-April, more than 1,700 people had died or gone missing this year in the sea that touches Europe, North Africa and the Middle East, according to the U.N. refugee agency.
But tens of thousands more migrants have reached Europe by crossing the Mediterranean. Some 36,390 people managed to reach Italy, Greece and Malta in the first few months of 2015, an agency spokeswoman Melissa Fleming said last month.
Italy's coast guard said that more than 3,400 migrants were saved in 16 operations off Libya's coast on Saturday alone.