The shooting took place in the village of Nyakova on the shores of Lake Albert in Congo’s eastern Ituri region.
“The soldier shot all the people around him. Twelve people died,” Ituri military spokesman Jules Ngongo said by telephone.
Investigative teams are looking for the soldier, who is a Congolese marine, and an investigation into the matter has been opened, he said.
Village chief Oscar Baraka Muguwa said 13 people were killed including 9 children.
It is unclear what led the officer to open fire. According to Muguwa, the soldier is worried that his son will be buried without him there.