Malian soldiers have been accused of killing an estimated 300 civilians in the central Malian town of Moura in late March.
According to a report from Human Rights Watch (HRW), the massacre took place over four days, with the vast majority of the victims being ethnic Fulanis.
Moura, which has a population of around 10,000, has been the epicenter of conflict-related violence and displacement since 2015, as Mali continues to battle a brutal jihadist conflict.
"The incident is the worst single atrocity reported in Mali's decade-long armed conflict," the HRW report said.
According to the rights group, soldiers arrived in the area on March 27 and exchanged gunfire with around 30 Islamist fighters. Malian soldiers and over 100 members of a foreign force — identified as Russians by several sources — were then deployed to Moura for a four-day operation.
"The soldiers patrolled through town, executing several men as they tried to flee, and detaining hundreds of unarmed men from the market and their homes," said HRW.
Mali's armed forces last week confirmed they had killed more than 200 Islamic militants during the operation and seized large quantities of weapons. The army issued a new statement on Tuesday, dismissing the "unfounded allegations" which it said were aimed at "tarnishing [its] image."
Harrowing accounts emerge
HRW officials spoke with 27 people to verify the claims, including witnesses, traders, and security analysts.
"The jihadists (armed Islamists) were buying and selling in the market and when the helicopters showed up, the jihadists started firing and the army fired back," one local trader said. "Everyone fled in panic, diving for cover."
"The villagers and traders tried to flee Moura, but by that time a few helicopters had landed and soldiers were everywhere. [...] All the traders who'd come to market were trapped in the village."
One resident, who witnessed multiple executions before he was finally freed by the soldiers on March 31, told HRW he "lived in terror, each minute, each second thinking it would be my turn to be taken away and executed."
"Even after being told to go, I feared it was a trap. As I walked away, slowly, I held my hand on my chest, holding my breath, and waiting for a bullet to pass through my body."
HRW's West Africa Director, Corinne Dufka, singled out the Malian authorities as the main culprits.
"Abuses by armed Islamist groups is no justification at all for the military's deliberate slaughter of people in custody," she said.
"The Malian government is responsible for this atrocity — the worst in Mali in a decade — whether carried out by Malian forces or associated foreign soldiers."
Russian mercenaries allegedly involved
Several witnesses and other sources identified the foreign soldiers as Russians.
A trader who had come to Moura from a nearby village to buy livestock told HRW: "White men speaking a bizarre language deployed throughout town. I wanted to flee but was afraid I'd be shot by the helicopter overhead. The white men arrested [me] and then took me to a place near the sand dunes where I found hundreds of others.”
Another trader said he and his two brothers "were in a friend's house drinking tea waiting for the market to get going when we heard shooting."
"Seven Russians approached, gesturing for us to get up. There were no Malian soldiers with them. They searched us and the house, then took us east of the village, near the river, where we found another 100 men."
As Mali's military-led government seeks support in its fight against Islamic militants following the withdrawal of French and European troops, there are increasing signals that Mali is turning to the Russian paramilitary group, Wagner, to help it win the war against the Islamist militants.
Russia says it has supplied what are officially described as military instructors to the West African country.
UN Secretary-General Antonio Guterres last week called on the Malian army and its "bilateral partners" — widely interpreted as an implicit reference to Russian mercenaries — to respect their international obligations amid growing concerns over human rights violations.
Mali's army reiterates jihadist threat
Mali's government has swiftly rejected claims that Malian soldiers have engaged in arbitrary killings.
In a statement, the vice president of Mali's Defense Commission of the National Transitional Council, Fousseynou Ouattara, said the allegations were formulated to undermine the army's achievements.
"The only goal is to blacken the victories that are being achieved by the Malian armed forces on the ground," he said. "Today, we have about 14,000 villages that have been liberated. There are more than 750,000 refugees and displaced persons who have returned to their homes."
HRW is calling for further investigations into the incident, as well as the punishment of offenders.
Former government minister Kassoum Tapo supports such a call.
"Instead of these atrocities, we should have investigated, verified and brought the opposite information," he told DW. "If it is false, deny it. If it is true, prosecute the possible perpetrators. War does not allow crimes to be committed. It does not authorize the violation of human rights."
Several unlawful killings of civilians and suspected Islamists have been documented since late last year. HRW has underlined that the killing of civilians will only push more Malians into the arms of the Islamist groups.
Armed Islamists have also killed scores of security forces since January, which rights groups are separately investigating in Mali's Menaka region.