The Rwandan-backed rebel group M23, Congolese Armed Forces and other armed groups committed serious rights violations in eastern Democratic Republic of Congo that may amount to war crimes, the UN Human Rights Office said on Friday.

M23, FARDC and other armed groups have been responsible for gross violations of international humanitarian law since late last year when fighting began in North and South Kivu, a new report by the UN Human Rights Office's Fact-Finding Mission said. 

"The atrocities described in this report are horrific," said UN High Commissioner for Human Rights Volker Türk in a statement, calling for accountability for victims.

M23 seized Goma, eastern Congo's largest city, in January and went on to make gains across North Kivu and South Kivu provinces. The fighting has killed thousands and displaced hundreds of thousands more this year, while escalating the risk of a full-blown war in a region rich in tin, gold and coltan. 

While multiple human rights bodies and the UN have accused parties in the Congo conflict of gross atrocities, this is the first UN report to find that those abuses may have constituted crimes against humanity.

The report found the ethnic Tutsi-led M23 carried out summary executions, torture, and enforced disappearances - which may amount to crimes against humanity.

The group also used systematic sexual violence, including gang rape, largely against women with an intent to "degrade, punish, and break the dignity of victims," the report stated.

M23 received training and operational support from the Rwandan Defense Forces, and there were credible allegations of the covert presence of RDF personnel within M23, the report said. 

Rwanda has repeatedly denied backing M23 and says its forces act in self-defence against Congo's army and ethnic Hutu militiamen linked to the 1994 Rwandan genocide. 

M23 has also previously denied allegations it has committed atrocities.

The report also found that the DRC armed forces and affiliated armed groups, such as pro-government militia fighters known as Wazalendo, had committed grave violations, including gang rape, the deliberate killing of civilians, and looting.  

Following mediation by Qatar, Congo and the rebels signed a declaration of principles on July 19 in which they vowed to start negotiating a peace deal in August. However, the two sides missed the deadline.