The U.S.-based group that operated the route said an aerial survey by helicopter showed no survivors.
The Beechcraft plane went missing in bad weather late Monday with two crew and 15 passengers on board, a spokeswoman for the U.N. Office for the Coordination of Humanitarian Affairs said.
It was located Tuesday 9.4 miles northwest of the airstrip at Bukavu in eastern Congo, its intended destination, Elisabeth Byrs told journalists in Geneva.
"Rescuers are on site," she said, adding that she had no confirmation of casualties. The identity of the passengers and crew was not immediately disclosed.
Air Serv International, a Warrenton, Virginia-based group, runs the twice-weekly aid delivery between Kisangani to Bukavu.
"Search and rescue efforts were initiated early this morning and visual confirmation of the downed aircraft was made," a company statement said.
The location was on steep ridge, it said.
Air Serv International describes itself as a not-for-profit aviation organization that supports humanitarian programs worldwide.
No Air Serv personnel were involved in the crash, said Suzanne Musgrave, a spokeswoman for the group told The Associated Press by telephone from Warrenton.
She said the plane was being flown by a commercial company, Cem Air.