One Moroccan UN soldier was killed and three others injured.
The attack took place in the southern diamond-mining town of Bangassou.
Christian militia have been attempting to seize a cathedral housing hundreds of displaced Muslims, who have been sheltering in the compound since a wave of ethnic killings in May.
The Red Cross said it found 115 bodies in the town after those attacks, when the rival religious groups clashed.
“They died in various ways: from knives, from clubs and bullet wounds,” a Red Cross representative told Reuters news agency at the time.
The wider conflict has killed thousands of people.
It broke out when mainly Muslim Seleka rebels ousted President Francois Bozize four years ago, provoking a backlash from the Christian militias.