Black descendents of certain ethnic groups are often born into slavery and spend their lives working as domestic servants or cattle herders for lighter-skinned masters.

The country has one of the highest rates of slavery in the world, with 1 in 100 people living as slaves, according to the 2016 Global Slavery Index. Activists say that anti-slavery laws are rarely enforced.

The two cases were brought by former slaves in the city of Nouadhibou.

In one case, the verdict pronounced on Wednesday was the result of a seven-year fight, said Salimata Lam of Mauritanian group SOS Esclaves, which assisted the victims.

The man sentenced to 20 years cannot be found, but the woman sentenced to 10 years was taken to prison, she said.

Earlier this year, the African Union urged Mauritania to issue harsher sentences for the crime.

"I think the trend is irreversible. You can't close your eyes to this situation," said Lam.

But there is still a long way to go, she added. Anti-Slavery International has helped file at least 40 cases from former slaves that are lingering in courts, said Sobik.

Mauritania has jailed more anti-slavery activists than slave-owners, and the repression of organisations fighting to end slavery is growing, rights groups said this month.