An attack by the Taliban on a school in Peshawar, Pakistan has been described by the UN Secretary-General as an act of horror, with no cause able to justify such brutality.
On Tuesday, more than 130 people were killed by the Taliban, the vast majority children.
Ban Ki-moon said the hearts of the world go out to the parents and families who lost loved ones.
No grievance, he said, can excuse such horror.
"It is an act of horror and rank cowardice to attack defenceless children while they learn. Schools must be safe and secure learning spaces. Getting an education is every child's right. Going to school should not have to be an act of bravery."
Mr Ban said the UN will continue to support the efforts of the Pakistani authorities in their fight against terrorism and extremism.
Other senior UN officials also condemned the attack.
The UN High Commissioner for Human Right, Zeid Ra'ad Al Hussein, said the Taliban have sunk to an all-time depth with what he called "this utterly despicable and incomprehensibly vicious attack".