BANGUI (Reuters) - As many as 100 people were killed last week in militia violence in southern Central African Republic and fighting fueled by ethnic and religious rivalries is spreading, the United Nations said on Tuesday.

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.

Clashes intensified on Monday in the town of Bria, about 300 km (180 miles) from the southern-eastern border town of Bangassou during the day, forcing about 1,000 civilians to seek shelter near the U.N. base, U.N. Secretary-General Antonio Guterres’s spokesman, Stephane Dujarric, said.

Medical charity Medecins Sans Frontieres’ (MSF) hospital in Bria had 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.

Meanwhile, the U.N. peacekeepers mission, MINUSCA, said the situation in the border town of Bangassou was “under control” after an attack by Christian militiamen at the weekend killed nearly 30 people and forced thousands to flee.

In a statement, Dujarric said unverifiable figures indicate that 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 U.N. Office for the Coordination of Humanitarian Affairs plans to lead an inter-agency fact-finding mission there.

In the town of Bangassou, at the border with Democratic Republic of Congo, 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 at the weekend.

“The worst is over,” the mission’s top general, Bala Keïta, told reporters in the capital Bangui. “We are holding the terrain and our men are going to continue search-and-sweep operations.”

According to the U.N. 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 violent attacks face justice after years of impunity.

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