Amnesty International says the Central African Republic (CAR) has witnessed a "horrific surge" in violence that is now clearly along sectarian lines.

Rebels in a volatile region of the country are raping women and killing their male relatives, Amnesty said in a report on Friday.

The report cited accounts by survivors in the eastern prefecture of Basse-Kotto, where a rebel group known as the UPC, an offshoot of the mainly Muslim Seleka rebel alliance, has been responsible for a series of attacks.

Joanne Mariner, the senior crisis response adviser at Amnesty, said civilians “are direct targets” in the conflict, condemning the performance of the UN peacekeeping force in protecting them. "If the UN's mandate in the Central African Republic is to mean anything, civilians must be better protected," she said.

The Central African Republic has been the scene of deadly violence due to political instability since March 2003, when General Francois Bozize overthrew former President Ange-Felix Patasse in a coup. The bloodshed deteriorated when the Seleka alliance overthrew Bozize from power.

Bozize's ouster caused deadly reprisal attacks between the Seleka rebels and the Christian militia known as anti-Balaka.

The United Nations has 12,350 troops and police on the ground to help protect civilians and support the government of President Faustin-Archange Touadera, whose election last year helped significantly contain the inter-communal violence.

International observers warn that the country is now approaching the levels of violence seen at the height of the conflict in 2014. The latest violence has engulfed parts of the country that were largely calm during the worst time of the earlier conflict.