The number of would-be migrants killed after their boats foundered late last month during desperate bids to depart Tunisia for Europe has risen to 24, a judicial source said on Wednesday.

Four boats capsized off the coast of the central Tunisian port city of Sfax on the night of April 22-23.

While 97 people were rescued, 24 have now been found dead, including women and children, said Sfax court spokesman Mourad Turki.

Authorities previously said they had recovered 17 bodies.

The morgue of Sfax hospital, the only one in the region to receive the bodies of migrants, has taken delivery of 92 such corpses in recent months, mostly of sub-Saharan Africans.

It is struggling to cope with the influx as "there is no space left" in a nearby cemetery for migrants, Turki said.

He added that around 40 bodies had been laid on the floor for lack of space.

Most Tunisian municipalities refuse to take charge of the corpses of those washed up.

Tunisia has become a key launchpad for migrants attempting to reach Europe on the world's deadliest migration route, the central Mediterranean.

Around 2,000 migrants went missing or died in the Mediterranean last year, up from 1,400 the previous year, according to the International Organization for Migration.