Alarm Phone, which provides support for people crossing the Mediterranean to the European Union, has reported 23 migrants missing.
A social media post explains that they set out from Boumerdès in Algeria on May 20 and were bound for the Balearics. The distance is some 300 kilometres.
The message posted around 8.30am on Saturday says relatives don't have any news from them. "We are all very concerned, where are they?" The relevant authorities have been informed.
The Caminando Fronteras collective, which defends the rights of migrants, recorded 1,037 deaths at sea last year on the Algerian migration route to the Balearics and the eastern coast of Spain.
So far this year, 1,754 migrants have arrived in the Balearics on 89 small boats.