The rebel March 23 Movement seized territory in North Kivu province in eastern Democratic Republic of the Congo. Former M23 leader Bosco Ntaganda surrendered to the International Criminal Court, where he's the subject of two arrest warrants for war crimes committed in DRC.
A report published Wednesday by the U.N. Joint Human Rights Office said human rights laws were violated during last year's conflict.
The investigation documented 135 cases of sexual violence by the Congolese military.
"The victims included 33 girls aged between 6 and 17," the report said.
The U.N. report said the integration of former rebels into the military and poor training were likely to blame for the incidents. M23 forces were accused of committing at least 59 acts of sexual violence.
The U.N. mission in DRC helped launched a judicial probe into the incidents in December, leading to the suspension of 12 military officers. Congolese authorities are conducting their own investigation.
U.N. special envoy for DRC Roger Meece said the United Nations was supporting Congolese efforts but stressed the probe should be independent and credible if that support is to continue.