A UN envoy on Thursday demanded access to the Malian village of Moura, site of an alleged massacre last month by local forces and suspected Russian fighters.
The UN's peacekeeping mission in Mali, MINUSMA, was able to fly over the site on April 3.
But its envoy for the Sahel nation, El-Ghassim Wane, told the Security Council an "integrated mission" had yet to receive a green light "despite extensive engagement with the national authorities."
Mali's army announced on April 1 that it had killed 203 militants in Moura, in central Mali, during an operation in late March.
However, that announcement followed widely shared social media reports of a civilian massacre in the area.
Human Rights Watch also said this week that Malian forces and foreign fighters killed 300 civilians in Moura in late March, in what it called "the worst single atrocity reported in Mali's decade-long armed conflict."
Several witnesses and other sources identified the foreign soldiers as Russians to HRW.
Russia has supplied what are officially described as military instructors to Mali, which has been battling a brutal jihadist conflict since 2012. The United States, France, and others, say the instructors are operatives from the Russian private-security firm Wagner.
Wane said he welcomed word from Mali that a military tribunal had opened an investigation into the events in Moura.
However, he added, "it is imperative that the Malian authorities extend the necessary cooperation for MINUSMA to have access to the site of the alleged violations, in line with its mandate."
On Wednesday, independent UN human rights expert Alioune Tine urged an independent and impartial investigation into the events.
In a statement, he called on the Malian authorities to allow the UN's MINUSMA force to perform the investigation.
"The findings must be made public and the alleged perpetrators brought to justice," Tine added.
Swathes of Mali lie outside of government control due to a brutal jihadist conflict that first emerged in 2012, before spreading to neighboring Burkina Faso and Niger.
Thousands of soldiers and civilians have been killed in the conflict, and hundreds of thousands have been forced to flee their homes.
Mali's under-equipped army has often been accused of committing abuses.
The country's ruling junta, which seized power in a military coup in 2020, routinely defends the rights record of the armed forces.