A Medecins Sans Frontieres (MSF) ship patrolling the Mediterranean discovered the bodies of 21 women and one man aboard a rubber dinghy off the coast of Libya on Wednesday.
The MSF ship MV Aquarius reached the scene in three hours following notification by the Italian coastguard, which had received a distress call at about 0800 GMT.
The MSF ship rescued 209 people, including 50 children who were in two dinghies, which had been sailing close together. An Italian naval vessel assisted in the effort.
— MSF International (@MSF) July 21, 2016
The 22 migrants found dead were lying in a pool of fuel, at the bottom of a dinghy.
"It is still not entirely clear what happened, but they died a horrible death. It is tragic," said Jens Pagotto, MSF Head of Mission for Search and Rescue Operations.
"It seems that water and fuel mixed together and the fumes from this might have been enough for them to lose consciousness," he told Reuters by telephone.
The survivors hailed mostly from Nigeria and Guinea, and were being brought to Sicily along with the dead and were due to reach the port of Trapani by Friday.
Italian authorities have reported a spike in the number of migrants who have left Libya this week on overcrowded boats in search of a better life in Europe, as people smugglers take advantage of calm seas and hot summer weather.
More than 2,500 people were rescued on Tuesday and one body was recovered, Italy's coast guard said. Almost 600 people were saved on Wednesday.
As of Monday, 79,861 migrants had arrived in Italy by sea so far in 2016 compared with 83,119 during the same period last year. Almost 3,000 migrants have perished or gone missing in the Mediterranean in the first seven months of this year, according to the International Organization for Migration.
"The survivors had been on the boat with the bodies of these women for hours on end. Many are too traumatized from what they have endured to be able to talk about what had happened," said Pagotto.
He said the survivors would be provided with a team of trauma specialists when they reached land.