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."
Children caught in conflicts faced grave violations such as killing, maiming, rape and other forms of sexual violence, attacks on schools and hospitals, and abductions, Skoog said.
He said Khartoum had made efforts to prevent recruitment of children into its armed forces since it signed an "action plan" with the UN in March 2016 to prohibit such recruitment.
Some rebel groups also had signed the plan but have failed to implement it fully, Skoog said.
"In spite of action plans signed by some of these groups, there are or have been... child soldiers deployed, which is a clear violation of their commitments," he said.
Skoog urged Khartoum to ensure that children who have given up arms be quickly reintegrated into society.
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.
"In any conflict children are the worst off, unfortunately that has been the case... in Sudan," Skoog said.