MADRID: Spanish police said Thursday that 25 migrants had likely died when their boat got into trouble during a harrowing eight-day journey from Mauritania to the Canary Islands, citing the survivors' testimonies.

The boat, which set sail from Nouakchott in Mauritania, was rescued off the island of El Hierro on Sunday with 48 survivors on board. Five of them were taken to hospital, including two minors.

In a statement, the police said the boat had experienced "terrible conditions" and that "according to survivors' testimonies, 25 people had likely died and their bodies were thrown over board".

Around 75 people had been on board when the boat left the Mauritanian coast on March 19, among them women and children, the police statement said.

"Within days, their water supplies ran out and they only had biscuits to eat" and seawater to quench their thirst, police said.

Three of the survivors, who were believed to be the boat's skippers, were arrested on suspicion of murder. Two remain in preventative custody, the statement said.

So far this year, 5,552 migrants have arrived on the Spanish islands after braving the perilous crossing from the African cost, double the figure for the same period in 2021, interior ministry figures show.

Migrant arrivals on the Atlantic archipelago have surged since late 2019 after increased patrols along Europe's southern coast dramatically reduced crossings to the continent via the Mediterranean.

At its shortest, the sea crossing from the Moroccan coast is around 100 kilometres (60 miles), but the distance from the Mauritanian shores is much greater -- more than 1,000 kilometres as the crow flies.

The Atlantic route is notoriously dangerous because of strong currents, with migrants often setting sail in overcrowded ramshackle boats which are extremely unsafe.