"Never in recent memory have so many children been subjected to such unspeakable brutality," UNICEF director Anthony Lake said.
"Children have been killed while studying in the classroom and while sleeping in their beds; they have been orphaned, kidnapped, tortured, recruited, raped and even sold as slaves.
"This has been a devastating year for millions of children," he said.
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Globally, 230 million children currently live in countries and areas engulfed in violent conflict, according to UNICEF.
The wars in Syria and Iraq, which have seen the rise of the Islamic State group, have left children in the clutches of increasingly brutal and extreme violence, the agency said.
More than 7.3 million children are affected by the war in Syria and 2.7 million in Iraq.
In Gaza, 54,000 children remain homeless from the 50-day war between Israel and Hamas militants that ended in August. Of the more than 2,000 dead from the fighting, 538 were children.