Leila Zerrougui, U.N. special envoy for children and armed conflict, said Seleka factions were conscripting child soldiers despite commitments made in November 2011.
"The same actors have been violating child rights with impunity for too long," she said in a statement. "We will continue to monitor the situation and if no progress is made, we will engage the (U.N.) Security Council on this matter."
The U.N. Children's Fund estimates that at least 2,500 children were involved in armed conflict in CAR before a major outbreak of violence there in December.
Zerrougui added that CAR armed forces urged the nation's youth to take up arms against rebel forces in the country.
"I urge the government to take its responsibility to protect children seriously, and to refrain from inciting violence," she said.
The Security Council last week expressed "serious concerns" over human rights abuses that resulted from the fighting between the rebel Seleka coalition and pro-government forces in CAR.
The Seleka coalition agreed to the terms of a cease-fire during peace negotiations with CAR officials in Gabon following their recent advance on CAR's capital Bangui. Their campaign was meant to pressure President Francois Bozize to step aside.