The Rte de l'Espoir (Road to Hope) from Nouakchott to Néma (around 1100km) is entirely tarred, giving a smooth (if very long) trip to the border with Mali. Check trusted security sources before travelling towards the border.
The first major town on the road to the Malian border from Nouakchott is Kiffa, an important regional trading centre and crossroads. You could also break up your journey at lively Ayoûn el-Atroûs, which is a good place to spend your last ouguiyas before crossing into Mali. The tarred road ends at the town of Néma, the jumping-off point for Oualâta.