Henrietta Fore told a Security Council meeting on children and armed conflict that it is ‘almost beyond comprehension’ that one of every four young people are caught in that situation, AP reported.
She pointed to children and young people whose lives are being shattered by conflicts, including in Yemen, Mali and South Sudan.
She also cited youngsters recruited to fight, killed by a land mine or an attack on their school, and "losing hope not only in their futures, but in the futures of their countries."
Sweden, which holds the Security Council presidency this month, organized the open meeting on the theme "Protecting children today prevents conflict tomorrow" and sponsored a resolution unanimously adopted by the 15 members to strengthen UN actions to ensure the care and safety of youngsters.
"We are not doing nearly enough to protect our children," Swedish Prime Minister Stefan Lofven, who presided at the meeting, said, stressing that "350 million children are affected by armed conflict today."
The resolution states for the first time that children recruited or caught up in armed conflict should be treated primarily as victims, he said.
It urges all countries "to consider non-judicial measures as alternatives to prosecution and detention that focus on the rehabilitation and reintegration for children formerly associated with armed forces and armed groups."
Lofven said the resolution also for the first time makes the point that the needs and vulnerabilities of girls and boys are different and stresses that access for all youngsters to education and physical and mental health care is essential.
It also sets out a framework to reintegrate children associated with armed groups or armed forces into society, which "places children as part of the solution, not part of the problem," he said.
Virginia Gamba, the UN special representative for children and armed conflict, said she is ‘profoundly shocked’ by the more than 21,000 violations of children's rights in 2017 recently reported by the UN, a significant increase from 2016.
"The majority of these despicable acts were perpetrated by armed groups although government forces and unknown armed actors played an important part," she said.
"Each and every one of them led to unspeakable suffering for children, families and entire communities."
Gamba said the level and severity of the latest violations demonstrate the need for united action "to change the tide of history," including by focusing on prevention and reintegration ‘to break cycles of violence’ against children.