A declaration of principles on a permanent ceasefire was signed by the parties as recently as July, with the goal of signing a peace agreement on August 18.
But despite this, the fighting has intensified since Friday around the town of Mulamba in the South Kivu province.
The "almost daily" attacks constitute a "deliberate and obvious violation" of the declaration, says the Congolese army's spokesperson Sylvain Ekenge in a statement.
On Monday, the fighting continued.
Both sides are using heavy weapons, says a resident who was interviewed by the news agency AFP over the phone.
More than two million people have fled the violence in eastern Congo-Kinshasa since January, according to a report from the UN's humanitarian organization OCHA that was released in July.
