Twenty migrants from sub-Saharan Africa were missing after their boat sank off Tunisia's coast in an attempt to reach Europe, a spokesman for the court that investigates such cases said on Saturday.

They were among 37 people aboard the vessel which had left from Tunisia's coast and then "sank on Friday afternoon", Faouzi Masmoudi, the court spokesman in Sfax city, said, adding that 17 people were rescued.

It was at least the sixth sinking since the beginning of March, in incidents which have left around 100 people dead or missing.

The latest case adds to the more than 14,000 migrants which Tunisia's coastguard said it has intercepted on their way to Europe between January and March.

That is more than five times the number from the first quarter of 2022.

Tunisia's shores lie only about 150 kilometres from the Italian island of Lampedusa. (AFP)