GENEVA (Reuters) - Deadly ethnic fighting in Central African Republic could descend into a much larger-scale conflict if nothing is done to disarm combatants and defuse tensions, a U.N. report said on Friday.

With a fifth of the population displaced since the mainly Muslim Seleka rebels ousted the president in 2013, provoking a backlash from Christian anti-balaka militias, U.N. peacekeepers are struggling to contain simmering violence.

The U.N. human rights working group on mercenaries and foreign fighters said it “strongly senses that the possibility of another armed conflict is likely, if foreign armed actors, along with local armed groups, are not effectively dismantled and suppressed.”

National security forces are too weak to tackle armed groups and counter the spillover from conflicts in neighboring countries, and U.N. military personnel, who number just over 10,000, have failed to convince locals that they can protect them, the report said.

Ugandan and U.S. forces pulled out earlier this year, declaring “success” against the Lord’s Resistance Army, a regional militia notorious for two decades of abducting children to use as fighters and sex slaves.

On Thursday, Amnesty International issued its own report detailing what it said was the systematic rape and murder of civilians in ethnic fighting.

“If the U.N.’s mandate in the Central African Republic is to mean anything, civilians must be better protected,” the rights group’s Joanne Mariner said.

U.N. peacekeeping chief Jean-Pierre Lacroix said the U.N. mission in Central African Republic (MINUSCA) had to make changes and was “slightly” short of the necessary personnel.

“We have currently a troop ceiling which we think is slightly under what we would need,” he said on Friday. MINUSCA’s mandate authorizes it to have 10,750 military personnel, a recent U.N. report said there were 10,098 on the ground.

“We are seeing a surge in ... very negative and antagonistic messages to the effect that ‘foreigners’ should be eliminated, putting one ethnic or religious component of this country against the other,” Lacroix told reporters in Geneva.

The working group report said the word “foreigners” was constantly used to label Muslims.