A U.N. human rights report found that more than 100 women were gang-raped allegedly by the Hutu rebel group the Democratic Forces for the Liberation of Rwanda, or FDLR, and members of the local militia Mai Mai in provinces bordering Rwanda.
"The United States will do everything we can to work with the U.N. and the (Congolese) government to hold the perpetrators of these acts accountable and to create a safe environment for women, girls and all civilians living in the eastern Congo," U.S. Secretary of State Hillary Clinton said in a statement condemning the attacks.
U.N. Secretary-General Ban Ki-moon said in a statement that he was "outraged" by the FDLR and Mai Mai atrocities committed in the Congo.
But Callixte Mbarushimana, the executive secretary for the FDLR, said from in a statement from Paris that his group was not behind the attacks.
The FDLR are "in no way involved in these odious actions and takes umbrage at the baseless accusations launched against them by the secretary-general of the United Nations," al-Jazeera quoted the statement as saying.
The executive secretary added that he had "serious questions" about the motives behind blaming the group.
The Hutu group, the United Nations said, was linked to the 1994 genocide in Rwanda.