As many as 100 people were killed last week in militia violence in southern Central African Republic, the United Nations has said, expressing "grave alarm" over the spread of fighting fuelled by ethnic and religious rivalries.

Among the victims were six UN peacekeepers, marking the deadliest month for the UN mission MINUSCA since it began in 2014.

The violence represents a new escalation in a conflict that began in 2013 when mainly Muslim Seleka fighters seized power and ousted then-president Francois Bozize, prompting reprisal killings from Christian anti-Balaka militias.

The UN high commissioner for human rights warned on Tuesday the violence in areas previously spared major bloodshed was "highly worrying".

"The hard-earned relative calm in [the capital] Bangui and some of the bigger towns in CAR risks being eclipsed by the descent of some rural areas into increasing sectarian violence, with defenceless civilians - as usual - paying the highest price," Zeid Ra'ad al-Hussein said.

Clashes intensified on Monday in the town of Bria, forcing about 1,000 civilians to seek shelter near the UN base, Secretary-General Antonio Guterres's spokesman, Stephane Dujarric, said on Tuesday.

Medecins Sans Frontieres' (MSF) hospital in Bria received 24 wounded people early on Tuesday as fighting continued, Frederic Lai Manantsoa, MSF's head of mission in the capital Bangui, said.

Casualty counts have been difficult to confirm because of the ongoing violence and remoteness of the locations.

"I don't know exactly how many but some were wounded and others died," one Bria resident said.

In a statement, Dujarric said unverifiable figures indicate up to 100 people may have been killed in three days of clashes from May 7-9 in the town of Alindao between anti-Balaka fighters and an ex-Seleka group.

Up to 8,500 people were displaced in the fighting, he said, and the UN Office for the Coordination of Humanitarian Affairs plans to lead an inter-agency fact-finding mission there.

Meanwhile, the UN peacekeeper mission said the situation in Bangassou, a town at the border with Democratic Republic of Congo, was "under control" after an attack by Christian militiamen at the weekend killed nearly 30 people and forced thousands to flee.

MINUSCA troops captured strategic sites after air strikes on Monday, the mission said. A total of 26 bodies have so far been identified there after fighting.

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"The worst is over," the mission's top General Balla Keita told reporters in the capital. "We are holding the terrain and our men are going to continue search-and-sweep operations."

According to the UN refugee agency, the violence in Bangassou sent an estimated 2,750 refugees fleeing across the border into Congo over the weekend.

In Bangui, hundreds marched to demand that the perpetrators of attacks face justice after years of impunity.

"We notice, unfortunately, that the violence continues to claim victims," said Evodie Ndemade, vice president of a victims' association. "Justice must be done now."