Four migrants were found dead on a boat heading for Spain's Canary Islands.
It is the latest in a string of attempted migrant crossings since the start of the year, in which at least 20 people have died.
Helicopters went to the aid of the makeshift boat on Sunday south of El Hierro island, after being spotted by a fishing boat, the Spanish coastguard said.
It was carrying 23 people, of which 19 were hospitalised. Three were said to be in a serious condition.
This year has seen a surge in the number of people attempting to make the crossing between Africa and the Canary Islands, a dangerous route due to strong currents.
Between 1 January and 15 March this year 3,436 migrants arrived in the Canary Islands, more than double the figure recorded last year.
Late last month two women and a man died on another overcrowded boat, with 41 people rescued.
A few days before that, a two-year-old Malian girl was reported dead on another boat of 52 migrants from Western Sahara, which was also rescued by the coast guard.