The Congolese army said Monday that it has launched an operation to reclaim territory lost after weekend clashes with rebels loyal to a wanted indicted war criminal.

Six fighters who previously belonged to the National Congress for the Defence of the People (CNDP) rebel group were killed Sunday after they attacked government forces in the eastern town of Mweso, regional army spokesman Sylvain Ekenge told AFP.

A senior military source who requested anonymity said the ex-CNDP fighters had taken control of the area surrounding Karuba and Mushaki, two areas near Mwesa in the eastern Kivu Nord province, which borders Rwanda.

"It's hills that separate Karuba and Mushaki and we are in the process of advancing hill by hill," the officer said.

The CNDP was integrated into the national army in 2009 following a peace deal with their leader Jean-Bosco Ntaganda, nicknamed "Terminator," who was indicted by the International Criminal Court in 2006.

But a group of several hundred CNDP loyalists, including roughly a dozen senior officers, defected earlier this month, citing unpaid salaries and inhumane living conditions among other complaints.

Those defectors attacked government troops in two separate raids on Sunday, sparking heavy clashes that sent civilians fleeing, Ekenge told AFP.

Hundreds of people, especially women and children, fled the fighting to the town of Sake where both the national army and the UN's peacekeeping force (MONUSCO) have bases and have found shelter in classrooms, an AFP reporter witnessed.

"We fled because we heard heavy firing and we walked six hours to get here," Jeanne, 22, told AFP in Sake.

The military said it could contain the new rebel threat.

"The situation is under control. The operations are continuing," against the defectors, Ekenge said and described the defectors as "a drop in the ocean" of the national army.

Before it signed a peace deal, the CNDP fought government forces the chronically-troubled Kivu Nord and at one stage threatened to capture the regional capital Goma.

Kinshasa was heavily criticised by some rights groups for dealing with Ntaganda, who is accused of recruiting child soldiers.

But the government has countered that Ntaganda's cooperation is needed to stablise the war-ravaged east.

Congolese President Joseph Kabila has faced heavy pressure to arrest Ntaganda and speculation that Kabila may act has intensified since the defections.

It however remains unclear if the defectors are still under Ntaganda's command.

"Up to now, he (Ntaganda) is still a general in the (army). I don't have a comment to make on that subject. It's the government who will handle it," Ekenge said.