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About 146 migrants are feared dead after their boat capsized after leaving Libya, according to a Gambian youth who was rescued following the disaster, the UN refugee agency said Wednesday.

The 16-year-old was barely conscious when he was spotted by the Iuventa, a ship belonging to German rescue organisation Jugend Rettet.

He was then brought onto a Spanish military ship participating in the EU's "Operation Sophia" to crack down on smugglers, before being taken to the Italian island of Lampedusa.

The vessel left on Sunday or Monday from Sabratha, northwestern Libya, with five children and several pregnant women among those on board, the teenager told a member of the UN High Commissioner for Refugees who met him at a hospital in Lampedusa.

Most of the passengers were from Nigeria, Mali and The Gambia, he said.

He said that the boat began taking on water a few hours after setting off, and that he survived by holding on to a fuel can.

"It shows that there may very well be shipwrecks we don't know about, because the boats sink without a trace," Flavio de Giacomo, a spokesman for the International Organization for Migration (IOM) said.

UNHCR said in a statement that it was "deeply saddened" by this latest tragedy, which "comes as a stark reminder of the vital importance of robust research and rescue capacities."

Since the beginning of this year, at least 590 migrants have died or gone missing along the Libyan coast, excluding this latest capsizing, the IOM estimates.

"Saving lives at sea must remain the key priority for all and UNHCR commends the action of the Italian Coast Guard in coordination with Frontex, the European Border and Coast Guard Agency," UNHCR's Volker Turk said.

Last week, the Spanish group Pro-Activa Open Arms discovered two empty and partially capsized dinghies, raising fears that hundreds of migrants could be dead, since smugglers often pack 120 to 140 people on such vessels, and sometimes many more.

But these incidents are not included in the IOM's estimates, in particular as one of the vessels may have been one that capsized in Libyan waters shortly before then, in which 54 people were rescued but 66 were missing.

The dangers have not slowed the surge in arrivals this year, however: The Italian coastguard says it orchestrated the rescue of more than 1,100 migrants off Libya between Tuesday and Wednesday morning.