The United Nations is "extremely concerned" that Mali has prevented its investigators from visiting a town where local troops and suspected Russian fighters allegedly killed hundreds of civilians, the world body said on Wednesday.
At least 300 men are believed to have been summarily executed during a March 27-31 raid on Moura, a town of about 10,000 inhabitants infiltrated by Islamist militants, according to a Human Rights Watch report.
Survivors said white mercenaries suspected to be Russians took part in the massacre that sparked international uproar and prompted the U.N. to open an investigation.
Mali has denied the allegations, saying it had conducted a professional operation to attack insurgents in Moura, and that it would carry out its own assessment.
"We are extremely concerned that Malian authorities have still not granted UN human rights investigators access," U.N. spokesperson Seif Magango said in a statement.
"Time is of essence to ensure accountability and prompt, effective justice for victims," he added.
Magango said unconfirmed sources suggest the death toll could be as high as 500, mostly civilians.
Soldiers also reportedly raped, looted and arbitrarily detained a number of Moura's inhabitants, the statement said.
The United Nations mission in Mali said separately on Wednesday that it was concerned by reports of more human rights violations committed by the Malian army, accompanied by a group of foreign military during a weekly market in Hombori in northern Mali on Tuesday.
The mission said on Twitter that it has opened an investigation and plans to visit the scene soon.
Mali is struggling to stem violent groups linked to al Qaeda and Islamic State that have gained ground and increased attacks over the past decade, spreading south and to bordering countries in West Africa's arid Sahel region.
A junta that toppled Mali's government in a 2020 military coup has sought the help of private fighters belonging to Russia's Wagner Group, accused of committing abuses in other countries and sanctioned by the European Union.
Both Mali and Russia have previously said they are not mercenaries but trainers helping local troops with equipment purchased from Russia.
The Russian government denies ties to Wagner.