The UN chief said in his annual report on Children and Armed Conflict, obtained by The Associated Press, that the United Nations verified 3 512 child casualties in Afghanistan, over 40% of the total and "the highest number ever recorded" in the country.
Guterres said the recruitment and use of children in conflict more than doubled in Somalia and Syria compared with 2015. And the UN verified 169 incidents affecting at least 1 022 youngsters in South Sudan - over 60% of them recruited and used by government security forces, he said.
The secretary general said the number of violations against children committed by extremist groups - al-Shabaab, Boko Haram, the Islamic State and the Taliban - totalled more than 6 800.
UN spokesperson Stephane Dujarric said in a statement that Guterres is "appalled" not only by the more than 8 000 youngsters killed and injured and the recruitment of children, but by sexual violence against girls and boys and attacks on schools and hospitals.
"A child killed, recruited as a soldier, injured in an attack or prevented from going to school due to a conflict is already one too many," said Virginia Gamba, the UN special representative for children and armed conflict.
The report contains a blacklist of government forces and rebel groups that recruit, use, kill, maim, rape, sexually abuse or abduct children in armed conflict or attack schools and hospitals.
For the first time this year, Guterres divided the list in two parts - one naming parties that have not taken any action to improve the protection of children, and one listing parties "that have put in place measures."
This year's list, which AP obtained and reported on Wednesday, was eagerly awaited because last year the Saudi Arabia-led coalition fighting rebels in Yemen was put on the blacklist but removed by then-secretary general Ban Ki-moon under intense pressure from the Saudis.
Guterres put the coalition on this year's list of parties that are taking action to protect children - a decision welcomed on Thursday by Human Rights Watch, Save the Children and the Watchlist on Children and Armed Conflict.
The secretary general said in the report that "the coalition's actions objectively led to the listing for the killing and maiming of children, with 683 child casualties attributed to this party, and as a result of being responsible for 38 verified incidents, for attacks on schools and hospitals during 2016".
The secretary general said there were 1 340 verified child casualties in Yemen - over 50% of the deaths and injuries caused by the US-backed coalition.
In talks with Saudi Arabia, Guterres said, its government told the UN the coalition took measures to reduce the impact of the conflict on children by changing rules of engagement and establishing a team to review all incidents involving civilian casualties and identify corrective actions.
"These initiatives are steps in the right direction," Guterres said. "Nevertheless, I urge the coalition to improve its approach since, despite these measures, grave violations against children continued at unacceptably high levels in 2016."
Dujarric said the secretary general spoke to King Salman of Saudi Arabia on Thursday. The country's UN ambassador has scheduled a press conference on Friday.
Human rights groups echoed Guterres' concerns.
Jo Becker, children's rights advocacy director at Human Rights Watch, said: "The coalition needs to stop making empty promises to exercise caution, take concrete action to stop these deadly unlawful attacks in Yemen, and allow desperately needed fuel and aid to reach those in need."
Save the Children Director Debra Jones said all parties fighting in Yemen "must sit up and take notice and take action, as must those countries that are supporting or arming them".
"The world must make sure the violations end," she said.
Eva Smets, Watchlist's executive director, said the Saudi-led coalition is now one of 64 parties listed for violating children's rights and the UN must work with all these parties "to ensure these horrific violations of vulnerable children are never repeated".
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