The investigation is focusing on an Indian unit accused of trading food for gold.
The fighting pits traditional militia known as the Mai Mai against dissident fighters loyal to dissident army Colonel Jules Mutebusi, whose rebel forces once briefly seized the eastern city of Bukavu in 2004.
De Brosses said six Mai Mai and three of the rival militiamen were killed.
It was not clear what sparked the fighting.
The mineral-rich eastern reaches of Congo, bordering Uganda, Rwanda and Burundi, are the most unstable in the country, with violence breaking out regularly that often pits rival militias against each other and leaves civilians dead.
UN peacekeepers helped end a wider 1998-2002 war in Congo that engulfed six neighbouring countries. A nearly 18,000-strong peacekeeping force currently in Congo is the UN’s largest peacekeeping operation.