The bodies of 21 women and one man have been found "in a pool of fuel" at the bottom of a rubber dinghy adrift near the Libyan coast, just hours after they had set sail for Italy, according to the humanitarian group Doctors Without Borders (MSF).
An MSF ship patrolling the central Mediterranean Sea came to the rescue of two dinghies that were sailing close together and managed to pull 209 people, including 50 children, to safety on Wednesday.
However, 22 people were found dead at the bottom of the first dinghy.
"When our team approached the first dinghy, they saw dead bodies lying at the bottom of the boat in a pool of fuel," said Jens Pagotto, MSF head of mission for search and rescue operations.
Speaking to the Reuters news agency, Pagotto said it was "still not entirely clear what happened, but they died a horrible death. It is tragic.
"It seems that water and fuel mixed together and the fumes from this might have been enough for them to lose consciousness."
The survivors, most of them from West African countries such as Nigeria and Guinea, were being brought to Sicily, along with the dead, and were due to reach the port of Trapani on Friday.
READ MORE: Italy plans a cemetery for refugees drowned at sea
Italian authorities have reported a jump in the number of refugees and 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 coastguard said. Almost 600 people were saved on Wednesday.
In June, the number of refugees and migrants who arrived in Italy by sea was nearly the same as during the same month last year, according to Barbara Molinario, the United Nations refugee agency's (UNHCR) communications officer in Italy.
Explaining that refugees and migrants are in desperate need of a "safe passage" to Europe, she told Al Jazeera: "Increasingly, we are seeing that people who are dying are the ones in the hull of the boat due to the lack of sufficient air. Many people have died of suffocation or from the heat."
REPORTER'S NOTEBOOK: 'Unprecedented' refugee rescue effort off Libya coast
Fleeing war and economic desperation, more than a million refugees and migrants reached European shores by boat in 2015, according to the UNHCR.
Following an agreement in March between the EU and Turkey, arrivals to Greece decreased drastically - more than 241,000 have made the dangerous journey so far this year.
The route from northern Africa to Italy, however, remains a concern. "It's much longer than the route from Greece," Molinario explained.
READ MORE: Life-jacket mountain a metaphor for Greece's refugees
"It takes at least three days and nights. The boats are bigger, but they are also very crowded, with 300 to 400 people on fishing boats made for 40 or 50 [people]."
At least 79,861 have made it to Italy by boat so far this year, while 83,119 made it during the same period last year. The UNHCR says at least 2,951 have died at sea when their boats capsized or are still missing.
MSF's Pagotto said the two dinghies had probably left Libya in the early hours of Wednesday and were picked up some 17 nautical miles east of the capital, Tripoli.
The Italian coastguard received a distress call at about 10am (08:00 GMT) and notified the MSF ship MV Aquarius, which took three hours to reach the scene. An Italian naval vessel also helped with the rescue.
"The survivors had been on the boat with the bodies of these women for hours on end. Many are too traumatised from what they have endured to be able to talk about what had happened," said Pagotto.
He said a team of trauma specialists would be on hand to help the survivors when they reached land.
Source: Al Jazeera and agencies