Two battalions of the Democratic Republic of Congo army are fighting each other close to the border with Uganda, reports from the area say.
Many rebel soldiers have been integrated into the army
The governor of North Kivu province, Eugene Serufuri, told the BBC that one group comprises soldiers drawn from former Mai Mai rebels.
The other battalion comprises soldiers integrated into the army from the Congolese Rally for Democracy (RCD).
The fighting is centred on the town of Rutshuru, 90km from Bukavu.
There are no details of casualties.
Thousands of refugees are reported to be crossing into Uganda.
The Congolese army, backed by UN peacekeeping forces, has recently been engaging rebel groups in eastern DR Congo in an effort to drive them out ahead of elections planned for later this year.
About 40 people have died in recent months, in fighting between Ugandan-based rebels and Congolese and UN forces.
The east has been out of the control of central government for many years.
Despite an the end to the civil war, bands of militia groups still terrorise civilians and use the rich minerals and timber of the region to finance their operations.
Several neighbouring countries were drawn into DR Congo's brutal five-year conflict in which 3m people were killed.