The troops' vehicle hit a mine and they were then ambushed by the group, a Malian security source told AFP, recounting what is a familiar jihadist tactic.
"We lost eight troops on Tuesday during a complex attack mounted by terrorists, between the localities of Nampala and Diabali," a Malian military source told AFP, adding that four other soldiers were injured.
Three Malian jihadist groups with previous Al-Qaeda links recently joined forces to create the "Group to Support Islam and Muslims" (GSIM), led by Iyad Ag Ghaly of Ansar Dine, and have already killed soldiers further east near the Burkina Faso border.
Barkhane's commander Francois-Xavier de Woillemont said in Ouagadougou the operation was not yet over, saying the final goal of such missions was "to get rid of the terrorists" who "harass local people and stop them living a normal life."
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It was the next step in operations undertaken in the zone "where we know that terrorists come to hide," he added.
Last July, 17 soldiers were killed in an attack on a military base in Nampala, which sits close to the Mauritanian border, claimed by the Islamist organisation Ansar Dine.
Residents of Niono, to the south of Nampala, told AFP they had seen reinforcements heading towards the area of today's attack.
The region where the soldiers were killed on Tuesday, Segou, is also rife with tensions between ethnic groups who dispute the use of land for farming and raising cattle.
Northern Mali fell to jihadist groups linked to Al-Qaeda in March 2012, and although these forces were driven out of key towns by a French-led military intervention the following year, the Islamists have now spread further south.