DAKAR, Senegal (AP) — Mali’s army and foreign soldiers suspected to be Russian recently killed an estimated 300 men — some of them suspected Islamic extremist fighters but most civilians — in Moura in central Mali, Human Rights Watch said Tuesday.

It is the worst single atrocity reported in Mali’s 10-year armed conflict against Islamic extremists, according to the rights group which said it interviewed several witnesses about the killings.

Russian fighters are believed to have shot dead most of those killed in Moura in late March, according to witnesses who identified the killers as white soldiers who did not speak French. Several hundred Russian mercenaries have been deployed in Mali to help fight the extremist rebels, the U.S. military confirmed in January.

In the Moura incident, Malian army troops and foreign soldiers in late March rounded up several hundred men and shot dead about 300 of them, burying many in mass graves and burning others, according to Human Rights Watch.

Mali’s defense ministry reported a similar incident, saying that in the last week of March it had killed 203 “terrorists” and arrested 51 others, acting on intelligence that armed extremists were meeting in Moura.

“Abuses by armed Islamist groups is no justification at all for the military’s deliberate slaughter of people in custody,” said Corinne Dufka, Sahel director at Human Rights Watch. “The Malian government is responsible for this atrocity, the worst in Mali in a decade, whether carried about by Malian forces or associated foreign soldiers.”

Most of those killed in Moura were from the Peul ethnic group, according to the rights group. Moura had been largely controlled by extremists linked to al-Qaida who taxed villagers and imposed strict Shariah law, according to residents.

“The Malian government should urgently and impartially investigate these mass killings, including the role of foreign soldiers,” Dufka said. “For such investigations to be sufficiently independent and credible, the authorities should seek assistance from the African Union and the United Nations.”

In its investigation of the killings in Moura, Human Rights Watch said its researchers spoke with 27 people including witnesses, traders, community leaders, foreign diplomats and security analysts.

Moura, a town of about 10,000 residents in the Djenné administrative area of central Mali, has since 2015 been at the center of the conflict with extremist rebels and has seen widespread violence, abuses by all sides and the displacement of large numbers of civilians.

The killings in Moura are part of a spike in violence in recent months by extremists linked to Al-Qaida in the Islamic Maghreb and the Islamic State in the Greater Sahara and by Malian government security forces. Extremists have also killed scores of Malian security force personnel since the beginning of 2022.

Extremists from the Islamic State in the Greater Sahara are alleged to have killed hundreds of civilians in March in Mali’s eastern Menaka region, which Human Rights Watch said it is investigating separately.

Since January, residents of central Mali have seen scores of foreign soldiers working with Mali’s military. The foreign fighters are believed to be Russian as they do not speak French, according to several witnesses who spoke to Human Rights Watch. Mali’s ruling junta announced in December that Russian trainers had arrived as part of a bilateral agreement with Russia. Shortly after Mali’s junta expelled French and other European soldiers who had been assisting the battle against extremists.

The U.S. State Department said it is concerned that “many reports suggest that the perpetrators were unaccountable forces from the Kremlin-backed Wagner Group,” while other reports say it was Malian forces.

“These conflicting reports illustrate the urgent need for the Malian transition authorities to give impartial investigators free, unfettered, and safe access to the area where these tragic events unfolded,” the State Department said in a statement from spokesman Ned Price.

It called on Mali’s transitional government to allow the U.N. mission in Mali to conduct a rigorous investigation. “Failure to provide a thorough and credible accounting of the facts and accountability will only serve to sow divisions in Malian society, undermine the credibility, legitimacy, and reputation of the FAMA (Mali’s military), drive communities into the hands of violent extremist groups, and create conditions for more violence.”