Fighting has intensified as the army in the Democratic Republic of Congo pushes into territory held by rebels loyal to the dissident General Laurent Nkunda.
The government troops have begun taking rebel-held ground
It follows the army's capture of rebel headquarters in the town of Mushake in North Kivu, east DR Congo on Wednesday.
A BBC correspondent with the army says troops are firing heavy artillery at rebel positions in the mountains.
General Nkunda says he is defending Tutsis against Hutu extremists and has rejected demands that he disarm.
The BBC's Arnaud Zajtman in the village of Kingi says government forces also have a position in the mountains and are hoping a further advance will enable them to attack the rebels on two fronts.
He said a new phase in the war began on Friday as the troops moved into territory held by the renegade general.
Rebel spokesman Rene Abandi said on Saturday that the insurgents had resisted an army advance and fought to protect civilian refugee populations in two rebel bastions, Kirolirwe and Kitchanga.
Our correspondent says that up to 15,000 people are living in a camp near Gen Nkunda's base and there are fears he may use them as human shields.
Four hundred thousand people have fled their villages in recent months and more civilians are now fleeing the area of the fighting.
The renewed violence has brought fresh fears for the safety of tens of thousands of people who have already been displaced.
UN forces, Congolese officials and humanitarian organisations are now drafting evacuation scenarios for civilians caught in the war zone, the French news agency AFP reports.
Gen Nkunda claims he is defending his own Tutsi community against Rwandan Hutu rebels responsible for the Rwandan genocide in 1994, who have been active in the east of DR Congo ever since.
The government had repeatedly warned him to stop his rebellion and integrate his men into the army.
The US has urged the rebel leader to go into exile in order to end the fighting, but his spokesman has denied reports that he had agreed.
The UN mission in DR Congo (Monuc) has been providing logistical support to the Congolese army, and on Tuesday said that as a last resort it would also "provide fire support" against the rebels.
The 15,000 UN soldiers in DR Congo are tasked with securing peace after a five-year conflict officially ended in 2002.