Migrants who survived say the boat was carrying 41 people when it set off for the Canaries in a particularly dangerous route due to strong currents.
At least 28 migrants and asylum seekers have been missing and are feared dead after their inflatable dinghy sunk off the coast of Spain's Canary Island.
Spain's coast guard said it rescued 13 people from the stricken vessel on Monday in the Atlantic Ocean some 105 kilometres (65 miles) south of the island of Gran Canaria.
But the rescued migrants told police the boat was carrying 41 people when it set off for the Canaries from Western Sahara, a coast guard spokesperson said.
According to Spanish NGO, Caminando Fronteras, which helps migrant boats in distress, the 13 who were rescued were all female, including a teenager.
In a separate incident, 44 migrants are believed to have drowned on Sunday off the southern coast of Morocco while trying to reach the Canaries. Moroccan authorities have not confirmed this information.
READ MORE:Thousands of Europe-bound migrants lost at sea in 2021: UN
Increase in sea crossings
The number of people attempting the sea crossing to the archipelago off the coast of northwest Africa has increased dramatically since late 2019 after checks on Mediterranean routes were tightened.
Spain says a total of 6,624 people arrived in the Canaries by sea between January 1 and April 30, 50 percent more than in the same period in 2021.
The route to the Canaries is particularly dangerous due to strong currents, with trips in often overloaded boats without enough drinking water taking more than a week to reach the archipelago.
Last year, over 40,000 migrants managed to reach Spain by sea, interior ministry figures show, almost the same number of arrivals as in 2020.
Caminando Fronteras said 4,404 migrants were lost at sea trying to reach Spain in 2021, more than double the figure from 2020.
READ MORE:Spain: Dozens of asylum seekers dead or missing after dinghy capsizes