The International Federation for Human Rights, known by its French initials FIDH, said more than 750 people have been reported killed in a month of fighting in CAR.
The U.N. Security Council in December backed French intervention to support an African Union peacekeeping force in CAR but FIDH said that's not enough.
"FIDH insists on the importance of the rapid establishment of a U.N. peacekeeping operation in the CAR," the organization said in a statement Wednesday. "Only such a mission can deploy the human, logistical and budgetary resources required to guarantee the safety of the civilian population in Bangui as well as throughout the CAR."
Seleka, a Sunni rebel coalition, toppled the CAR government in March. The group was accused of committing war crimes and retaliation from a Christian militia known as anti-balaka has given the conflict a religious tone.
FIDH President Karim Lahidji said the situation will spiral out of control without an international intervention.
"The civilian population is totally unsafe," he said. "The worst is feared if the international community doesn't react accordingly."
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