"During the night, we counted (many) attacks" on both civilians and on military positions, Captain Guillaume Ndjike, an army spokesman in the strife-torn North Kivu province, told AFP.
Ndjike blamed the raids on Mai-Mai tribal militias and local self-defence groups often formed on ethnic lines, as well as Rwandan Hutu rebels from the Democratic Forces for the Liberation of Rwanda (FDLR).
In the bloodiest single attack, seven people were shot dead or hacked to death with machetes in the town of Kibirizi, he said, confirming an earlier report by local officials.
Another four civilians were killed in Kashalira while a soldier and a civilian were killed in Kahunga, the army official said.
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A young Hutu was also shot dead by Mai-Mai fighters in Buleusa, according to a delegate from the provincial governor in the town, Roger Bihango.
Kahunga, Kashalira and Kibirizi are located in the Rutshuru territory, while Buleusa is part of Walikale. The FDLR is highly active in these two territories in the southern part of North Kivu.
North Kivu has been the scene of repeated clashes for nearly two decades, and hundreds of thousands of people have been displaced.
Some older members of the FDLR are accused of taking part in the massacres of about 800,000 people in neighbouring Rwanda, mainly from the Tutsi minority, over three months in 1994 before they were routed by the rebel army of current President Paul Kagame.
Several FDLR chiefs face international accusations of war crimes or crimes against humanity, but the rebels have carried out no significant cross-border operations inside Rwanda since 2001.
The Nande accuse Congolese Hutus of abetting the FDLR. The Hutus, in return, say they are looking for land where they can settle for farming and accuse the Nande of trying to expel them.