"Seventeen people, all of them from the (Muslim) Fulani minority, were killed on Monday by young gunmen claiming to be from the (mostly Christian) anti-balaka" militia near the town of Bambari, an officer from the African Union force told AFP on condition of anonymity.
"Some of the bodies were mutilated and burnt by the assailants," he said, adding that the killings sparked reprisal attacks in the town by the mostly Muslim Seleka ex-rebel force.
Anti-balaka chiefs contacted by AFP in the capital Bangui denied that their fighters were responsible for the violence, with one of them, Brice-Emotion Namsio, putting the blame on "out-of-control youths acting on their own... Who have been stepping up these attacks in the region."
The Central African Republic has seen more than a year of unrest, with violence between ex-Seleka rebels and the largely Christian militias leaving tens of thousands dead and about a quarter of the population of some 4.5 million displaced.
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"The international community must support African, French and soon UN forces in putting an end to these crimes, protect civilians and bring those responsible for these crimes to justice," FIDH president Karim Lahidji said.
The central Bambari region, which lies some 400 kilometres (250 miles) from Bangui and where the ex-Seleka rebels have established their new headquarters, has seen several outbursts of violence this month.
Bambari was also the scene of a brutal January attack on Centralafrican radio journalist Blanche-Elisabeth Olofio by ex-Seleka rebels. Olofio died of her injuries over the weekend.