The United Nations has called on authorities in the Democratic Republic of Congo to carry out criminal investigations into acts of violence committed by the former March 23 movement (M23) rebels in the eastern part of the African country between April 2012 and November 2013.

"In the light of documented breaches of human rights and international law, it is recommended that Congolese authorities open sweeping, thorough and fair judicial investigations on the crimes committed by civilians and fighters in M23 in North Kivu province," the Office of the United Nations High Commissioner for Human Rights (OHCHR) said in a statement released on Thursday.

The UN accuses the M23 members of killing, raping and torturing hundreds of people, stressing that the actual number of victims in the Democratic Republic of Congo could be considerably higher.

It says some of the cases could be considered to be "war crimes and crimes against humanity."

The M23 rebels defected from the Congolese army in April 2012 in protest over alleged mistreatment in the Armed Forces of the Democratic Republic of Congo (FARDC). They had previously been integrated into the Congolese army under a peace deal signed in 2009. The group was defeated last November by government forces and UN troops.

Several armed groups are active in the eastern Congo and are fighting for control of the country’s vast mineral resources, such as gold, the main tin ore cassiterite, and coltan (columbite-tantalite), which is used to make many electronic devices, including cell phones.

About 2.9 million people have resettled in Congo and some 500,000 have crossed into neighboring countries, including Rwanda and Uganda.

Congo has faced numerous problems over the past few decades, such as grinding poverty, crumbling infrastructure, and a war in the east of the country that has dragged on since 1998 and left nearly six million people dead.