Preliminary reports indicate a total of 28 people have been shot dead in Nouna town of the province of Kossi in Burkina Faso.
Twenty-eight bodies were found in northwest Burkina Faso over the weekend, the government has said, noting an investigation was under way as speculation grew over who may be responsible.
Attacks targeting the security forces and civilians have increased in recent months, especially in northern and eastern regions bordering Mali and Niger, which are also battling militants.
"The government was informed of an incident at Nouna... during the night of December 30-31," a government statement said late on Monday.
Preliminary reports "indicate 28 people killed," it said, adding that an investigation had been opened and urging calm.
READ MORE: Suspected militants target local volunteers supporting Burkina Faso army
The CISC (Collective of Communities against Impunity and Stigmatisation) organisation denounced the violence as "reprisals targeting civilians" by armed civilians claiming to be members of the VDP auxiliary force who "freely engage in organised looting and targeted abuses of civilian population."
One of the world's poorest countries, Burkina has already seen two coups by disgruntled army officers last year that have put the military in power.
Since 2015 the West African country has been grappling with an insurgency led by militants affiliated with Al Qaeda and Daesh which has killed tens of thousands and displaced around two million people.
READ MORE: Casualties as militants strike Burkina Faso villages