The ouster of Central African Republic leader Francois Bozize by the Seleka rebels early this year and the subsequent unrest has sparked religious infighting in the volatile country.
After the coup in March, a Christian self-defense militia called anti-balaka has been engaged in running battles with the Islamic Seleka militias.
"We are seeing the seeds of a profoundly dangerous development between communities.
I want to become a rebel and kill members of Seleka.
"It's a tinderbox that can ignite into something very, very big and very, very bad," the UN director of humanitarian operations, John Ging told reporters.
Seleka fighters have raided, looted, and attacked CAR Christians, and in response, forces loyal to Bozize began to kill Muslims.
Residents of the capital city of Bangui say attacks have become common.
Homes, markets, communities have been attacked by both parties as religious tensions grow.
"I want to become a rebel and kill members of Seleka. We suffered too much. Muslims are our enemies," a resident told the BBC.
Towns have become deserted and government buildings are now in a sorry state.
Displaced people have sought refuge at Catholic missions, and according to eyewitness reports, more than 35,000 Christians have been displaced.
Last week, an estimated 1,000 people, mainly women and children, arrived at a Catholic mission in the town of Bossangoa.
Tensions have risen so much that Muslims attend Friday prayers, with their Kalashnikovs.
The African Union has 2,100 peacekeepers in CAR, but promises to increase the number to 3,600 have not been kept.
Last week, the UN approved a special 250-strong military force to protect UN workers in the country.