Burkina Faso insurgents killed 35 mostly female civilians on Tuesday after attacking a military outpost in northern Soum Province, and about 87 militants and local security forces were killed in the clash, authorities said.
President Roch Marc Kabore declared two days of national mourning in the west African country in response to the attack.
The incident followed an attack on a mining convoy in November killed nearly 40 people - victims of an Islamist insurgency that has ignited ethnic tensions and rendered large parts of the country ungovernable this year.
Militants attacked a military detachment in Soum province on Tuesday morning. After several hours, troops repelled them and seized a large number of weapons and motorbikes, the army said in a statement.
"As they fled, in a cowardly way the terrorists killed 35 civilians of whom 31 were women," the government said in separate statement. It said 80 militants and seven members of the security forces were killed in the earlier fighting.
Burkina was once a pocket of relative calm in the Sahel region, but its homegrown insurgency has been amplified by a spillover of jihadist violence and criminality from its chaotic northern neighbor Mali.
Attacks over the past year have killed hundreds and forced nearly a million people from their homes.