The Afghan government has made progress on the issue, said Leila Zerrougui, the UN representative for children and armed conflict. But she said the Afghan Local Police - government-allied groups that often operate as independent militias and are widely seen as unprofessional and corrupt - are major perpetrators.
The Taliban, who have been battling the government for over 15 years, mainly recruit children in provinces bordering Pakistan and other areas where the fighting is fiercest, she said.
"They are not going to school, they are deprived of access to health. They are targeted by armed groups and they are prevented from having hope for the future."
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.
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The report said insurgents "have been training and deploying children for various military operations" in Afghanistan, including making and deploying bombs.
It said the Taliban had recruited child fighters since the 1990s, but had expanded the practice with new madrassas and training centres in the country's north.
The Taliban condemned the HRW report in an emailed statement today, saying that it banned the recruitment of children as fighters.
Despite the downward trend marked by the UN, the government is still struggling to curtail the practice.