Seleka, a Sunni rebel coalition that is formally disbanded, toppled the CAR government in March. The anti-Balaka group, a Christian militia, has since taken up arms against their Sunni rivals.
OCHA said in an operational update Friday about 20 percent of the CAR population has fled their homes because of persistent violence. In Bangui, more than 500,000 people have become displaced since Dec. 5.
"The security situation remains tense, with serious risk of deterioration in and outside Bangui and serious consequences for humanitarian assessments and response," the United Nations said in a statement.
The U.N. Security Council in December authorized a French military intervention to help support an African Union peacekeeping mission in CAR. U.S. and British forces are assisting the French operation in a non-combat role.
Babar Baloch, a spokesman for the U.N. refugee agency, said much of the CAR civilian population was living in a state of fear.
"People are hiding in the bush, fearing fresh attacks," he said in a statement Friday from Geneva, Switzerland.