The Mi-8 chopper belonged to the Russian airline UTair and went down in difficult terrain in the Democratic Republic of Congo, the Russian law-enforcement source told RIA Novosti.
UTair was in the DRC under a U.N. contract. The aircraft was flying in bad weather between Shabunda and Bukavu when contact was lost.
"According to preliminary information, a Russian Mi-8 helicopter crashed in the mountainous terrain at an altitude of 2,700 meters, some 20 kilometers (12 miles) southwest of the town of Bukavu," the source said.
The fate of the crew was officially unknown Sunday and the crash site was difficult to reach.