At least 12 civilians were killed and 30 wounded, including children, in an attack northwest of the Central African Republic capital Bangui.
In a statement on Tuesday, Amy Martin, head of the UN Office for the Coordination of Humanitarian Affairs in Bangui, said the attack occurred on Monday.
The UN said Christian armed groups known as "anti-balaka" appeared to have targeted Peul herders, who are mostly Muslim.
Martin said the incident was one of a number in recent days, including fighting on Tuesday just 45km north of Bangui.
"It looks much more organised just than community defence groups," she said, of the armed group that has formed to counter abuses by former rebels, known as Seleka.
BINUCA, the UN office in Bangui said unidentified armed men had targeted Boali, about 60 miles from the capital during the weekend.
While not providing details on who was responsible, the UN warned of tensions between communities leading to "a climate of increasing violence" in the country.
This comes on the heels of a UN Security Council vote on Thursday to dispatch French reinforcements to restore order in a country that has slipped into chaos since mainly Muslim rebels seized power, leading to tit-for-tat sectarian violence.
Prospere Ndouba, an adviser to Michel Djotodia, the rebel chief who is now serving as interim president, said anti-Seleka youth backed by supporters of ousted President Francois Bozize were responsible for the Boali attack.
Around 460,000 people, a tenth of the population, have fled the sectarian violence since the Seleka rebel coalition, a loose alliance of warlords, seized power in March.
France has begun dispatching reinforcements to its former colony, where it will have around 1,000 troops to support an African force that has so far struggled to contain the fighting.