While the international community is focused on the M23 rebellion, other armed groups have taken advantage of the security vacuum in eastern Congo, and killed over 260 people since April this year, a United Nations report said Wednesday.
"In April 2012, army desertions and the subsequent creation of the M23 armed group led the Congolese army to focus on efforts to contain this new rebellion. Many armed groups have taken advantage of the security vacuum left by the redeployment of army units to expand their own areas of influence," said the report.
A series of six investigations by the U.N. has determined that the Raia Mutomboki and Nyatura armed groups were responsible for most of the 75 massacres in which at least 264 people died, including 83 children.
The actual number of victims is likely to be higher as not all massacres were investigated.
The Raia Mutomboki - meaning "Angry Villagers" in Swahili - is a Congolese armed group that was created in South Kivu in 2008 to defend the local population against the exactions of the FDLR, a Hutu militia of fighters who helped perpetrate the 1994 Rwandan genocide and who have operated in eastern Congo since then.
For several years the Raia Mutomboki restricted their operations to South Kivu, but in early April, they became active in the Walikale and Masisi territories of North Kivu as the Congolese army left these two areas to redeploy in the Rutshuru territory and fight the M23.
"If you want to put an end to red ants attacks, you have to go to their nest. When we finished operations here in (South Kivu) we thought that the FDLR can come back. If we neutralize Masisi we know we can put an end to the FDLR," said Juriste Kikuni, the Raļa Mutomboki second in command, to the Associated Press in September.
The Raļa Mutomboki's attacks have become increasingly vicious, killing FDLR fighters and Hutu civilians indiscriminately and hacking their victims to death with machetes and spears, or burning them alive in their homes.
"We attacked Ufamandu (a village in Masisi) because the population was with the FDLR, and we, we attack the FDLR and their allies," said Kikuni.
Children and elderly, unable to flee fast enough, are often the principal victims of the attack, U.N. investigators found.
In retaliation, the opposing Nyatura armed group has carried out similar attacks on Tembo civilians, one of the two ethnicities that make up the Raia Mutomboki.
The ethnic dimension of the massacres raises grave concerns, the U.N. report says, as conflicts in eastern Congo over the past 15 years have largely concentrated along ethnic lines.
"The systematic human rights violations committed by these armed groups, including the slaughter of so many children, are the most serious we have seen in recent times in the DRC," said U.N. High Commissioner for Human Rights Navi Pillay.
In response to the attacks, the Congolese army has deployed several units to Masisi, with the support of the U.N. peacekeeping mission, known as MONUSCO, over the period of July to September, but several NGOs and the local population feel that not enough efforts are being made.
"While it is clearly necessary that civilians attacked by M23 be protected, this should not happen at the cost of innocent lives - mainly women and children - elsewhere in the region. The population feels abandoned by MONUSCO forces which has failed in its mandate," said a Jesuit Refugee Service staff member in Masisi.
In addition to calling for an increase in security measures to protect the population, the U.N. report recommends that Congolese authorities take in account the human rights violations detailed in the report when integrating armed groups in the future.
The Congolese army is currently in the middle of negotiations with the Nyatura armed group to integrate its fighters into the national army to try to stabilize the region.
The Congolese army has a history of integrating in its ranks armed groups responsible for human rights violations.
General Bosco Ntaganda, one of the leaders of the M23 rebellion that started in April, was already wanted by the International Criminal Court when he was initially integrated in the Congolese army in 2009.
Prosecution for war crimes and crimes against humanity remains largely nonexistent in Congo. The Raia Mutomboki's top commander, who goes by the name of Eyadema, was arrested in Kindu in September but no judicial procedure has begun against him yet.