The Afghan government had made progress on the issue, said Leila Zerrougui, special representative of the UN Secretary General for children and armed conflict.
However, she added that Afghan Local Police, who often operate independently from central oversight and have been widely criticized as unprofessional and corrupt, are major perpetrators of child recruitment among Afghan forces.
Insurgent groups recruited more children in areas where the fighting is fiercest, she said. The Taliban, who have been fighting the government for over 15 years, mainly recruit children in provinces bordering Pakistan, she added.
The Pakistani authorities have denied connections with the insurgent group, and have levelled similar accusations at Afghanistan's government.
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Zerrougui spoke to reporters a day after the New York-based Human Rights Watch released a report accusing Taliban forces of boosting the number of children in their ranks since the middle of last year, in violation of international laws against the use of child soldiers.
The report said insurgents "have been training and deploying children for various military operations" in Afghanistan, including making and deploying bombs.