"These migrants died of thirst on the way to Algeria," the source, who asked not to be identified, said late yesterday.
The group, a man, a woman and five children aged between six and 12, "were abandoned by a smuggler" and "all died of thirst", he added.
Online news site Air-info, which is based in northern Niger, said the eight were part of a larger group headed for Algeria, and that four men who had been rescued were found "completely dehydrated".
Several thousand Niger citizens living illegally in Algeria have been expelled since 2015 following a deal between the governments of both countries.
Niger in 2015 adopted tough legislation against human trafficking that sets jail sentences of up to 30 years for smugglers.