Twelve were killed on Wednesday as they went towards a blast that killed a soldier, a local official told AFP. When did these 12 did not return, six members of their community set out to look for them Thursday, only to be killed by the same group.
The bodies of the first 12 had been booby-trapped with explosives, a security source added, though it was not clear whether this was how the second group had died.
The deadly series of events were unleashed when a Malian army vehicle transporting rations exploded near the village of Tigula, killing a soldier, on Wednesday.
"Having heard the explosion, villagers went towards the scene... when terrorists intervened to execute them," said a local official from the Mondoro commune of which Tigula is part.
Six others set out on Thursday to look for the first group, but were also "killed by terrorists," the official said.
Another municipal source and a security official confirmed the toll of 18 civilians.
Mali has been embroiled in conflict since Islamist militias seized the north of the country in 2012 before being pushed back by French troops in 2013.
A peace agreement signed in 2015 by the Bamako government and armed groups has failed to restore stability.
An upsurge of inter-communal violence in the country's centre and south has claimed some 600 lives, including 160 members of the Fulani herding community killed in the village of Ogossagou in March.
The massacre brought tens of thousands of protesters out on the streets of Bamako in April, causing the prime minister and his entire cabinet to resign.
A new government is in the process of being formed.