Last year Sudan freed 21 child soldiers who were captured in Darfur in 2015 during fighting between government forces and the rebel Justice and Equality Movement group.
The United Nations has said that some armed groups fighting in Sudan still deploy child soldiers, but acknowledged Khartoum's efforts to prevent child recruitment into its forces.
Olof Skoog, chairman of the UN Security Council's working group on children and armed conflict, said armed groups including rebel factions were not implementing international regulations on children in conflict.
"We have made very strong appeals to the armed groups that they fully subscribe to international law when it comes to respecting children," Skoog told reporters in Khartoum during a trip by the working group to Sudan.
He said getting information on children in Sudan's conflict zones of South Kordofan and Blue Nile was particularly difficult, but that his group had met former child fighters who painted a worrying picture.
"They have told me that they know of some other children who are still locked into these armed groups and with the rebel groups," he said.
"I can't give you any number but that makes me worried."