Twenty-eight bodies were found in northwest Burkina Faso over the weekend, the government said, and rights activists blamed a volunteer militia created to support the army's battle against jihadis.
Attacks targeting the security forces and civilians have increased in recent months, especially in northern and eastern regions bordering Mali and Niger.
"The government was informed of an incident at Nouna ... during the night of December 30-31," a government statement said late Monday.
Preliminary reports "indicate 28 people killed," it said, adding that an investigation had been opened and urged calm.
But a rights group called the Collective of Communities against Impunity and Stigmatizations (CISC) pointed the finger at the Volunteers for the Defense of the Fatherland (VDP), a civilian auxiliary force that supports the military in its 7-year-old fight against jihadis.
The public prosecutor in Nouna, Armel Sama, said in a statement that "most of the victims, all of them males, were shot dead."
The landlocked West African country is one of the poorest and most volatile nations in the world.
Since 2015, it has been grappling with an insurgency led by jihadis affiliated with al-Qaida and the Islamic State group that have killed tens of thousands and displaced around 2 million people.
The VDP, set up in December 2019, is made up of civilian volunteers who are given two weeks of military training and then work alongside the army, typically carrying out surveillance, information-gathering or escort duties.
Experts have long worried that the poorly trained volunteers are easy targets for the jihadis and may also dangerously inflame ethnic friction without proper controls.
The CISC said the weekend events in Nouna had begun with a reported "terrorist attack" on a local VDP headquarters.
Armed men then carried out "deadly attacks in reprisal," it said. Victims said the assailants were VDP who were members of a traditional hunting community called the Dozo, according to the CISC.
CISC Secretary Daouda Diallo called on the authorities to pay "special attention" to the situation.
"Armed terrorist groups exploit these kinds of transgression to attract recruits among the public," Diallo warned.
Three incidents of abduction and extrajudicial killings allegedly involving Dozo or VDP had occurred in the runup to the events at the weekend, CISC said.
Government spokesman Jean Emmanuel Ouedraogo said the weekend killings "unfolded at a time when Burkina Faso has launched an operation to mobilize the entire population in a united action in the fight against terrorism."
In November, the authorities, backed by a patriotic campaign, launched a drive to recruit 50,000 VDP, and 90,000 signed up.
The government is "fundamentally opposed to all forms of abuse or violations of human rights for whatever reasons," the statement said.
The VDP has taken the brunt of losses suffered by the security forces in the face of the jihadi campaign.
Hundreds of volunteers have died, especially in ambushes or explosions caused by improvised explosive devices (IEDs) planted along roadsides.
The escalating toll among the army, police and VDP unleashed two military coups last year, launched by officers angered at failures to stem the bloodshed.