A U.N. envoy says up to 700 children have been killed or maimed in Iraq since the beginning of the year, "including in summary executions."

Leila Zerrougui, the secretary-general's special representative for children and armed conflict, told the U.N. Security Council on Monday she is "appalled" at the killing of civilians, including children, by the Islamic State militant group which now controls a large swath of Iraq and Syria.

She said the extremist group has ordered boys as young as 13 years old to carry weapons, guard strategic locations or arrest civilians, and has used other children as suicide bombers.

Zerrougui said her office has also received report of militias allied to the Iraqi government using children in the fight against extremists from the Islamic State group.