Forty-one migrants including three children are feared dead after being shipwrecked last week in the Mediterranean, UN agencies said, citing four survivors brought to the Italian island of Lampedusa Wednesday.

Their metal boat overturned in bad weather during the night of Thursday to Friday after setting off from the Tunisian port of Sfax, said a joint statement from the UN agencies for refugees, children and migration.

The survivors -- a 13-year-old boy on his own, a woman and two men -- drifted for days before being rescued by a merchant ship on Tuesday, they said. They were finally brought to Lampedusa by the Italian coastguard on Wednesday.

In a separate statement, the Italian Red Cross, which manages the migrant reception centre on the island, said the four were generally in good health.

They reported to be from Ivory Coast and Guinea, and were unrelated to the missing migrants, it said.

The shipwreck is one of several deadly incidents reported in recent days after a period of bad weather.

Officials on Monday had reported that 16 migrants died in shipwrecks off the coasts of Tunisia and Western Sahara.

And on Sunday, the UN's migration agency, the International Organization for Migration (IOM), said at least 30 people were missing after two shipwrecks off Lampedusa.

The tiny island, located just 90 miles (around 145 kilometres) from Tunisia, is the first port of call for many migrants heading from North Africa to Europe.

But many of them do not survive, making the Central Mediterranean migrant crossing the world's deadliest.

More than 1,800 people died attempting the route so far this year, according to IOM figures from Friday -- more than double the fatalities in the same period last year.

- 'Low-cost boats' -

The four survivors told the Red Cross they had survived by floating on inner tubes. A statement from EU border patrol agency Frontex suggested at least some other people from their boat had survived.

A Frontex plane on Tuesday morning spotted "a metal boat with four people on board" in waters covered by Libyan search and rescue, the agency said.

The boat was "adrift" so Frontex raised the alarm and the four were rescued by a merchant vessel, before being transferred to an Italian coastguard vessel.

IOM press officer Flavio Di Giacomo said the migrants' boat would have been ill-equipped for the kind of bad weather seen in the Central Mediterranean in the past week.

"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 told AFP on Wednesday.

"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."

People traffickers who sent migrants to sea in such conditions are "more criminal than usual... totally without scruples", he added.

- 'Sent to slaughter' -

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.

It had been "sending them to slaughter with this sea", he added.

Almost 94,000 migrants have landed on Italy's shores so far this year, according to interior ministry figures -- up from almost 45,000 in the same period last year.

In their statement, the UN agencies on Wednesday renewed their call for "coordinated search and rescue mechanisms" in the Central Mediterranean.

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".