The UN has sent a team of investigators to central Mali where it says more than 150 people were killed over the weekend.
A UN spokeswoman said the massacre in the village of Ogossagou marked a significant surge in violence across communal lines by so-called self-defence groups.
They have become increasingly aggressive since 2015 when jihadists started carrying out attacks in the area.
The UN says 600 people, including many women and children, have been killed in the Mopti region over the last year.
President Ibrahim Boubacar Keita has called on the military to be constantly mobilised and said Mali was in a state of war.