About 40 fighters from the Popular Front for the Renaissance of Central African Republic (FPRC) armed with AK-47s and rocket propelled grenades had gathered in the town, but U.N. forces intervened to prevent them carrying out an attack, it said.

The action was in keeping with the peacekeeping forces' mandate to protect civilians and its aim to "prevent a war" between the militia and the rival Union for Peace in Central African Republic (UPC) rebels, it added.

"The toll of this air operation is not yet known," mission spokesman Vladimir Monteiro said. "MINUSCA (the U.N. mission) warns the coalition (FPRC) against any attempt to bring war to Bambari."

Monteiro told Reuters in an email that the operation had "engaged one helicopter," but gave no further details.

The FPRC and UPC are former members of the mainly Muslim Seleka rebel alliance that united to oust then-President Francois Bozize in 2013, sparking backlash from Christian "anti-balaka" militias and plunging the country into tit-for-tat ethnic and sectarian killing.

But they fell out over competition for territory and control of illicit tax revenues. The FPRC are now allied with the anti-balaka militias.

UPC rebels killed at least 32 people in clashes with the FPRC in December, according to Human Rights Watch (HRW).