At least 58 people, including women and children, were killed after a boat carrying dozens of migrants capsized in the Atlantic Ocean off the West African nation of Mauritania, the UN's migration agency said.

The perilous sea passage from West Africa to Europe was once a major route for migrants seeking jobs and prosperity.

The sinking is one of the deadliest incidents since the mid-2000s when Spain stepped up patrols and fewer boats attempted the journey.

The boat carrying at least 150 people was low on fuel while approaching Mauritania before it capsised, the International Organization for Migration (IOM) said.

It said 83 people swam to shore. The survivors were being helped by Mauritanian authorities in the northern city of Nouadhibou, IOM said.

Survivors said the boat left The Gambia on November 27.

IOM's Leonard Doyle said the vessel was unseaworthy and overcrowded when it overturned.

"It speaks really to the callousness of the smugglers who of course have made their money and disappeared into the wilderness. That's the problem here, people are being exploited, people are looking for a better life," Doyle told Al Jazeera.

An unknown number of injured were taken to hospital in Nouadhibou.

There was no immediate statement from authorities in The Gambia, a small West African nation from which many migrants set off in hopes of reaching Europe.

'Horrible story'

Al Jazeera's Mohamed Vall, reporting from Mauritania's capital Nouakchott, said the military police discovered the survivors - most of whom came from The Gambia - and that is when the extent of the tragedy became clear.

"It's a very horrible story and one of the deadliest incidents in regard to migrants trying to cross the Atlantic Ocean or the Mediterranean Sea towards Europe this year," said Vall. "It's been confirmed that women and children were on that boat and some of them lost their lives."

Although home to some of the continent's fastest-growing economies, West Africa is struggling to generate enough jobs for its growing population of young people. 

Doyle said the survivors would likely be returned to their home countries.

"We can imagine that they're deeply traumatised. People will need some medical care and our staff will need to establish their origin and try to help them return in the most dignified way as possible. The tragedy in all this is there is no happy solution for people who take these routes," he said.

Despite the Gambia's small size, more than 35,000 Gambian migrants arrived in Europe between 2014 and 2018, according to the IOM.