Leila Zerrougui, the U.N. envoy, addressed delegates in Geneva during a meeting for the U.N. Human Rights Council.
She said conflict spilling across parts of central African was ensnaring children.
"The relapse into conflict in the Central African Republic and the eastern Democratic Republic of the Congo had affected the most vulnerable, and children previously separated from armed groups had been re-recruited," she said in a statement published Tuesday.
Rebels from the March 23 Movement are battling against Congolese forces, backed by U.N. peacekeepers, in eastern DRC. M23 is suspected of recruiting child soldiers.
More than half of the estimated 66,000 people who've fled DRC violence to neighboring Uganda are children, UNICEF said.
In CAR, the rebel Seleka coalition is accused of war crimes in its early 2013 move to topple the government. The United Nations estimates CAR's entire population of 4.6 million people, about half of them children, are in need of some form of humanitarian assistance.
"The recruitment of child soldiers and the increasing number of child victims of armed conflicts around the world were appalling," Zerrougui said.