A UN spokesman said here Wednesday that at least 264 civilians, including 83 children in southern Masisi in the eastern part of the Democratic Republic of Congo (DRC) had been arbitrarily executed by armed groups in more than 75 attacks on villages between April and September of 2012. The UN report published by the UN Joint Human Rights Office ( UNJHRO) on Wednesday was based on six investigative missions and more than 160 victims and witness interviews. It concluded that \" the Raia Mutomboki armed group, with allied Mayi Mayi groups, was responsible for most of the killings,\" said UN spokesman Martin Nesirky at a daily news briefing here. \"Many victims were hacked to death with machetes while others were burnt alive in their homes,\" Nesirky added. The Raia Mutomboki group allied with the community-based Mayi Mayi groups led ethnic attacks against the Hutus, an ethnic group living in Rwanda, eastern DRC and Burundi. Another opposition fraction in the region, the Nyatura group, was found responsible for other human rights violations, including killings against mainly the Tembo ethnicity which were \"sometimes carried out in collaboration with the Democratic Forces for the Liberation of Rwanda (FDLR),\" said Nesirky. These groups have increased power and devastation in the region as international attention was drawn to rebellion group M-23 which has also caused human rights violations since April this year and had UN Secretary-General Ban Ki-moon openly speak out against them.The \"important progress in tracking down the FDLR made early in 2012 by the Congolese army has been reversed since their redeployment to M23-threatened areas,\" UNJHRO said in the press release. Many armed groups \"have taken advantage of the security vacuum left by the redeployment of army units to expand their own areas of influence, often carrying out violent attacks against civilians and exacerbating inter-ethnic tension, already heightened by the M23,\" it added. \"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,\" UN High Commissioner for Human Rights Navi Pillay said in a news statement released on the official High Commissioner website. \"The Congolese authorities must take immediate measures to protect civilian populations and to combat the persistent impunity which only serves to embolden the killers,\" it said.