A top militant and three others were killed in Central African Republic when a U.N. helicopter fired on fighters advancing towards the town of Bambari, a rebel group said on Sunday.

The UN's mission known as MINUSCA shot at fighters from the Popular Front for the Renaissance of Central African Republic (FPRC), on Saturday after they crossed a "red line" it had set north of the town, said spokesman Vladimir Monteiro.

"We were looking to prevent war in Bambari," he said, referring to the town about 250 km (155 miles) northeast of the capital Bangui.

A death toll had not yet been established, he added.

The FPRC is the largest group within a mostly Muslim rebellion formerly known as Seleka which ousted then-President Francois Bozize in 2013.

Months of reprisal killings between Muslims and Christians ensued, resulting in thousands of deaths, until elections last year ushered in relative calm.

"MINUSCA used air strikes yesterday against our fighters near Ippy," said Azor Kanite, the FPRC's deputy commander. "Our top commander (Joseph Zonduko) and three civilians were killed by the bombings," he added.

Despite multiple attempts to promote dialogue and disarmament between fighters in the aftermath of last year's polls, flare-ups in the former French colony are common.

Since November, FPRC fighters have been fighting the mostly Fulani Union for Peace in Central Africa (UPC) around Bambari, killing dozens and displacing around 20,000 people. 

The U.N.'s top genocide official said in November that the FPRC had singled out ethnic Fulani in the town of Bria, carrying out house-to-house searches, killing, looting and abducting residents.