The Soviet-made Ilyushin-76 belonged to Trans Air Congo and appeared to be transporting merchandise, not people, said an aviation official.
The plane was coming from Congo's second-largest city, Pointe Noire, and tried to land during heavy rain, he said.
Ambulances rushed to the scene in the Makazou neighbourhood, located near the airport, but emergency workers were hampered by the lack of light in the capital, which like so many in Africa has a chronic shortage of electricity.
"At the moment, my team is having a hard time searching for survivors in order to find the victims of the crash because there is no light and also because of the rain," Congolese Red Cross head Albert Mberi said.
He said that realistically, they will only be able to launch a proper search today, when the sun comes up.