Three children were among the dead. No bodies have been recovered.

Four people, three men and a woman from the Ivory Coast and Guinea, were rescued from the metal boat, which set off from Sfax in Tunisia on 3 August.

Three of the four claim to be minors, while the fourth is an adult male.

UN agencies, however, reported there was only one minor among them.

The Red Cross said the four survived using inner tubes and managed to climb onto another empty vessel nearby.

The four survivors are being cared for on Lampedusa, with the Red Cross saying their conditions were good and that they would be transferred to the Italian mainland soon.

Large wave flipped boat

The boat, which was carrying a total of 45 passengers, capsized and sank just six hours into its journey.

A large wave flipped the vessel, causing everyone on board to be thrown into the sea.

Photos released by the Sea-Watch humanitarian rescue group showed the four survivors waving for help from the boat and making their way to a commercial tanker, the Maltese-flagged Rimona.

The migrants rescued by the Rimona were then transferred onto an Italian coast guard vessel which brought them to the Sicilian island of Lampedusa on Wednesday, said Sea-Watch's Paul Wagner.

Mr Wagner said a plane from the European border agency Frontex had spotted the migrants in distress, and sent out a mayday call on open radio, prompting Sea-Watch to fly over the location reported.

Sea-Watch then spotted the people and proceeded to inform the closest merchant vessel, which was the Rimona.

According to Sea-Watch, the survivors were spotted inside the Libyan search and rescue zone around 40 miles from Tunisia and about 125 miles from Lampedusa.

Other boats sank in same area recently

It comes shortly after two boats capsized in the same area, prompting a rescue operation of more than 55 people.

It is not clear if the incidents are linked.

On Sunday, another 11 bodies were recovered from a shipwreck near Sfax, with more than 40 people still missing from that boat.

More than 93,000 migrants have arrived in Italy so far this year, compared with 44,700 in the same period a year before, the country's interior ministry said.

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The International Organisation of Migration said the latest shipwreck brings to more than 1,800 the number of people dead and missing in the central Mediterranean, the most active and dangerous migration route in the world.

Juan Matías Gil, MSF Search and Rescue Representative, said: "These kinds of tragedies are happening daily in the Mediterranean and at the European borders, right in front of everyone's eyes.

"More and more people have been taking the sea to find refuge over the last few days and weeks, and several deadly shipwrecks have occurred, particularly along the Tunisian route.

"These are not mere incidents; they demonstrate that European governments' efforts to externalise border policies are not set in the favour of people on the move, but it will make their journeys more dangerous and precarious."