Human Rights Watch estimated that about 300 people were killed in total in the town of Moura in volatile central Mali, with the vast majority of the victims being ethnic Fulanis.
Malian troops and suspected Russian mercenaries have allegedly executed around 300 civilian men over five days during a military operation in a central town.
The killings took place between March 27 and 31 in Moura, a rural town of around 10,000 inhabitants in the Mopti region, Human Rights Watch (HRW) said in a report on Tuesday.
"The incident is the worst single atrocity reported in Mali's decade-long armed conflict," HRW said.
The reported executions sparked condemnation from the United States, the European Union, France and Germany, who have all called on the Malian government to allow for an independent investigation.
The West African country's army on Saturday said it had killed more than 200 fighters in the operation in Moura after it received information about a meeting between rebel groups.
It added that it would investigate any allegations of rights abuses.
A Mali military spokesperson did not immediately respond to requests for comment on the report by New York-based HRW.
Mali: Massacre by Army, Foreign Soldiers https://t.co/4wuQzlnqp7— Human Rights Watch (@hrw) April 5, 2022
Nineteen witnesses told the HRW that Malian and Russian-speaking soldiers arrived by helicopter and exchanged two rounds of gunfire with the fighters, during which rebels, soldiers and a few civilians were killed.
The troops then deployed through the town, summarily executed several men then gathered hundreds of unarmed others from their homes and took them to the bank of a nearby river, the witnesses told HRW.
Many were traders from surrounding villages who came to attend the town's weekly livestock market. Some were infiltrated militants, the witnesses told HRW.
The men were held for five days under the sun and arbitrarily selected for execution by gunfire during the night. Bodies were piled into three mass graves, HRW said.
The operation allegedly involved over 100 Russian-speaking men, according to multiple security sources who spoke to HRW.
Mali's transitional government, which took power in a 2020 military coup, is battling the militancy with the help of private military contractors belonging to Russia's Wagner Group.
Both Mali and Russia have previously said they are not mercenaries but trainers helping local troops with equipment bought from Russia.