At least two children have been beheaded in the Central African Republic's capital as violence sinks to a "vicious new low", the UN has said.
About 16 children had been killed in Bangui since 5 December and an increasing number of them are being recruited into armed groups, it added.
The violence has mostly pitted Christian and Muslim militias against each other.
The UN said children were being directly targeted in revenge attacks.
The CAR is ruled by Muslim ex-rebel leader Michel Djotodia, who seized power in March forcing then-President Francois Bozize, who came from the majority Christian population, to flee into exile.
About 1,000 people have been killed in tit-for-tat clashes in Bangui in December and about 370,000 people, nearly half the city's population, have been displaced, aid agencies say.
There were "unprecedented levels" of violence against children, the UN children's agency, Unicef, said in a statement.
One of the children who had been beheaded had also been mutilated, while another 60 had been injured in fighting since 5 December, it said.
"More and more children are being recruited into armed groups, and they are also being directly targeted in atrocious revenge attacks," said Souleymane Diabate, Unicef's CAR representative.
"Targeted attacks against children are a violation of international humanitarian and human rights law and must stop immediately. Concrete action is needed now to prevent violence against children."
Last month, the UN said the number of child soldiers in CAR had more than doubled to 6,000 as fighting escalated.
Many Muslim fighters belong to the former rebel group, Seleka, and Christians to militias called "anti-balaka", named after the local Sango language word for machete.
The African Union has sent nearly 4,000 troops to CAR while France, the former colonial power, has also deployed 1,600 soldiers there in an attempt to quell violence.
Many foreign governments have been evacuating their citizens from CAR.