More than 40 people have died in a migrant shipwreck off the Italian island of Lampedusa.
Four survivors told rescuers the vessel had set off from Sfax in Tunisia, a hot spot in the migration crisis.
The metal boat overturned during bad weather last Thursday night, according to a joint statement from the UN agencies for refugees, children and migration.
The seven-metre vessel was carrying 45 people when it began to take in water after reaching the open sea. The survivors told the coastguard that the boat sank a few hours into its journey after being overwhelmed by a giant wave.
The four managed to survive the shipwreck by floating on inner tubes, before reaching another boat at sea.
They were eventually rescued by a merchant ship in the Straits of Sicily and then transferred to an Italian coastguard vessel, which took them to Lampedusa on Wednesday.
The Sea-Watch charity rescue group said one of its surveillance planes spotted them being rescued.
“They said they were among the few aboard [the boat] with a life jacket, and [after the shipwreck] they remained in the water until they found another empty boat,” Sea-Watch said in a statement.
Alessandra Filograno, a spokeswoman for the Italian Red Cross, confirmed four survivors arrived at the Lampedusa reception centre on Wednesday morning: two men, a woman and a 13-year-old unaccompanied boy from Ivory Coast and Guinea. The charity said the four were in generally good health.
The migrants arrived in Lampedusa exhausted and in a state of shock, and were due to be questioned by police, local prosecutor Salvatore Vella said. They were believed to have spent several days adrift with no food or drinking water.
A press officer for UN migration agency, the International Organization for Migration, Flavio Di Giacomo said Wednesday the migrants who died in the latest tragedy would have had little chance given the bad weather.
"Sub-Saharan migrants (leaving from Tunisia) are forced to use these low-cost iron boats which break after 20 or 30 hours of navigation," he said.
"With this kind of sea, these boats capsize easily. It is very likely that there are many more shipwrecks than those we know about, that is the real fear."
He said people traffickers who send migrants to sea in such conditions are "more criminal than usual... totally without scruples".
The local prosecutor's office has opened an investigation into the incident.
It came after the coastguard recovered the bodies of a woman and a toddler on Sunday, following two shipwrecks off Lampedusa.
There were 57 survivors of the shipwrecks, with more than 30 believed to be missing. Survivors said the boats had set off from Sfax and sank in rough seas.
One boat was carrying 48 passengers, of whom 43 were rescued by the coastguard. The second vessel had 42 people on board, 14 of whom were rescued.
An investigation into Sunday's shipwrecks has been opened in Agrigento, on the Italian island of Sicily.
Agrigento's chief of police Emanuele Ricifari said the traffickers would have known bad weather was forecast.
"Whoever allowed them, or forced them, to leave with this sea is an unscrupulous criminal lunatic," he told Italian media at the weekend.
He added that it had been "sending them to slaughter with this sea".
A source said it was unlikely that the four survivors picked up on Wednesday were from one of these two earlier shipwrecks.
Lampedusa is Italy's southernmost island and is closer to Africa than the Italian mainland. The island is about 130km from Sfax.
It has become a key European entry point for migrants, mainly originating from Libya, since the early 2000s, and has seen its migrant holding centre repeatedly overcrowded with new arrivals this summer.
Last year the Italian navy relocated 600 asylum seekers from the tiny island, which is home to about 6,000 people, after its refugee reception centre became overwhelmed with new arrivals, and photos circulated of filthy conditions.
Italy has registered about 93,700 migrant arrivals by sea so far this year, according to Interior Ministry data last updated on Monday, compared with 44,700 in the same period of 2022.
Most of those arriving are from Guinea, Ivory Coast, Egypt and Tunisia.
Italian Premier Giorgia Meloni, whose right-wing government includes the anti-migrant League party, has galvanised the European Union to join it in efforts to coax Tunisia to crack down on smuggling operations, but the boats continue to set off.
In their statement, the UN agencies for refugees, children and migration on Wednesday renewed their call for "coordinated search and rescue mechanisms" in the Central Mediterranean, and for countries to increase resources to better meet "their responsibilities".
They also called for more safe legal routes for migrants and refugees into Europe, "to avoid people having to resort to dangerous journeys in search of safety and protection".