The Sunni militant onslaught in Iraq has created "an extremely volatile and dangerous situation for children," a top UN official warned Tuesday.

There were "disturbing reports" of the recruitment of child soldiers and other "grave violations" against minors in the conflict, said Leila Zerrougui, the UN special representative for children and armed conflict.

The jihadist group leading the Iraqi offensive had since 2011 been on the United Nation's black list in particular because of its attacks against schools, Zerrougui noted.

Her comments came as she presented to journalists an annual report on her department's activities.

The report details rights violations against children in 23 conflict situations around the world.

It accuses seven national armies and 50 armed groups of using child soldiers -- including Central African Republic, the Democratic Republic of Congo, South Sudan and Syria.

Among recent developments was the blacklisting of Nigerian Islamist group Boko Haram, responsible for the kidnapping of hundreds of school girls in the country's north.

In contrast, Chad was removed from the UN list after taking a series of measures since 2011 to halt the recruitment of child soldiers in its army. Yemen pledged to do the same last month.

In South Sudan, however, the conflict that broke out in December 2013 between supporters of President Salva Kiir and his former vice president Riek Machar "has erased most of the progress accomplished to protect children since the country's independence," lamented Zerrougui.

She also noted a renewed increase in attacks against schools and hospitals, especially in Syria, Iraq, Nigeria, Afghanistan, the Central African Republic, and the Democratic Republic of Congo.