Around 50 migrants are feared to have drowned in waters off the Moroccan coast, the Spanish coastguard has said.
Rescue services managed to retrieve three "exhausted and disorientated" men from a half-sunk rubber dinghy near Spain's Alborán Island.
It is believed many more were originally on board.
A coastguard spokeswoman said they had likely drowned, adding that rescuers were still surveying the water where the boat was found.
The survivors are aged between 17 and 25, and from sub-Saharan Africa. They have been taken to hospital in Almería, southern Spain.
The coastguard said in a statement that the boat "had been drifting for several days after leaving the northern coast of Morocco".
If the missing are confirmed dead, this will be the deadliest sea crossing of 2017 in this part of the Mediterranean.
According to the International Organization for Migration (IMO), 6,464 people reached Spain after crossing the Mediterranean between 1 January and 25 June 2017.
A further 60 died while attempting the journey.
The IMO says they are among at least 2,247 people who have died or are missing after trying to cross the sea into Europe via Spain, Italy or Greece this year.