The Italian island of Lampedusa has lost 41 migrants in a shipwreck, survivors told local media.

Four survivors of the disaster told rescuers they were on a boat that had left Sfax in Tunisia and sank on its way to Italy.

Originally from Guinea and the Ivory Coast, the survivors reached Lampedusa on Wednesday.

So far this year, more than 1,800 people have died crossing from North Africa to Europe.

An investigation into the tragedy has been opened by the local public prosecutor, Salvatore Vella.

Survivors – a 13-year-old boy, two men, and a woman – told rescuers they were on a boat with 45 people, including three children.

According to them, the boat, which was about 7m (20ft) long, left Sfax last Thursday, but sank within hours after being struck by a big wave. There are only 15 individuals believed to have been wearing lifejackets, but this may not have been enough to save them.

According to the Italian Red Cross and German charity Sea-Watch, the four survived the shipwreck by floating on inner tubes and life jackets until they found another empty boat at sea, in which they drifted for several days before being rescued.

Despite exhaustion and shock, the four survivors arrived in Lampedusa with only minor injuries, according to Adrian Chiaramonte, the doctor who treated them.

It was the story of the tragedy that really struck us, he said.

According to them, a first ship ignored them after encountering them.

An hour later, they were spotted by a helicopter, and an hour after that, an oil tanker picked them up.”

On Sunday, the Italian coast guard reported two shipwrecks in the area, but it is unclear if this vessel was among them.

Migrants would have had little chance of survival, according to the International Organization for Migration (IOM).

According to IOM spokesman Flavio Di Giacomo, Sub-Saharan migrants [leaving Tunisia] are forced to use these cheap iron boats that break after 20 or 30 hours of navigation. With such a rough sea, these boats capsize easily.

A port city about 80 miles (130 km) from Lampedusa, Sfax, is a popular gateway for migrants seeking a better life in Europe.

More than 2,000 people have been rescued from Lampedusa by Italian patrol boats and charity groups in recent days.

Black Africans have faced a wave of racism in Tunisia in recent months, and boat departures have increased.

There have been more than 17,000 deaths and disappearances in the central Mediterranean region since 2014, making it the world’s most dangerous migrant crossing.

In a bid to curb “irregular” migration, the EU signed a deal with Tunisia worth $118 million (£90 million).

Migrants will be returned, borders will be strengthened, and smuggling will be stopped.