By Agence France-Presse
The UN's refugee agency (UNHCR) said it was "deeply alarmed" after the Golfo Azzuro, a boat operated by Spanish NGO Proactiva Open Arms, reported the recovery of the bodies close to the drifting, partially-submerged dinghies, 15 miles off the Libyan coast.
"We don't think there can be any other explanation than that these dinghies would have been full of people," Proactiva spokeswoman Laura Lanuza told AFP. "It seems clear that they sunk."
She added that the inflatables, of a kind usually used by people traffickers, would typically have been carrying 120-140 migrants each.
"In over a year we have never seen any of these dinghies that were anything other than packed."
Lanuza said the bodies recovered were African men with estimated ages of between 16 and 25. They had drowned in the 24 hours prior to them being discovered shortly after dawn on Thursday in waters directly north of the Libyan port of Sabrata, according to the rescue boat's medical staff.
Vincent Cochetel, director of the UN refugee agency (UNHCR)'s Europe bureau, said NGO boats patrolling the area had been called to the aid of a third stricken boat on Thursday afternoon, raising fears others may have perished on what Proactiva called "a black day in the Mediterranean."
Despite rough winter seas, migrant departures from Libya on boats chartered by people traffickers have accelerated in recent months from already-record levels.