The refugees and migrants were rescued on the Mediterranean route from Libya to Italy in the last few days, IOM says.
About 250 people are feared to have drowned in the Mediterranean after two partially submerged rubber dinghies were found off the coast of Libya.
Laura Lanuza, a spokeswoman for the Spanish NGO Proactiva Open Arms, said its boat, Golfo Azzurro, recovered five floating bodies on Thursday close to the dinghies, about 30km off the Libyan coast.
She said the vessels would typically carry 120-140 people each.
“We don’t think there can be any other explanation than that these dinghies would have been full of people,” Lanuza told AFP news agency.
“In over a year we have never seen any of these dinghies that were anything other than packed,” she added.
Lanuza said the bodies recovered were African men aged between 16 and 25. They drowned apparently in the 24 hours prior to them being discovered in waters directly north of the Libyan port of Sabratha.
— Open Arms ENG (@openarms_found) March 23, 2017
A spokesman for Italy’s coastguard, which coordinates and participates in rescues, confirmed the five bodies were on board the rescue boat, which will remain in the area in case of any emergency calls.
“It is a harsh reality check of the suffering here that is invisible in Europe,” Proactiva Open Arms wrote on Facebook.
Despite rough winter seas, migrant and refugee departures from Libya on boats chartered by people traffickers have accelerated in recent months from already record levels.
More refugees died in the Mediterranean over the first nine weeks of 2017 compared with the same period in 2016, the International Organization for Migration (IOM) says. From January 1 to March 9 this year, at least 521 people drowned while attempting to cross the rough waters compared with 471 in the same period a year ago.
There were about 5,000 recorded deaths in all of 2016.
More than 6,000 people have been rescued on the central Mediterranean route from Libya to Italy in the last few days, the IOM said on Tuesday. So far this year 16,248 migrants have arrived in Italy, up from 13,825 in the same period last year.
Last month, European leaders signed a controversial plan to help stem the flow of African migrants to Europe. As part of the deal, the European Union will give $215m to Libya’s fragile government to step up efforts to stop boats in the country’s territorial waters.
The EU will also provide support for the setting up of “safe” camps in Libya and the voluntary repatriation of refugees willing to return to their countries of origin.
The plan has been criticised by several aid groups, however, that say leaders have abandoned humanitarian values and misrepresented conditions in Libya, where the UN-backed government of Fayez Serraj has only shaky and partial hold on the country.
Since February’s agreement was made, consultations between the Libyan government, representatives of the interior ministers of Italy and other European countries have been ongoing.
On Monday, Serraj asked for and additional $864m in military, rescue and emergency equipment to curb illegal migration across Libya’s border into Europe.