The Save the Children charity has said that two million children trapped inside Syria are at constant risk of malnutrition, disease and trauma.
In a report published today, the organisation says the conflict in Syria has led to the collapse of childhood for millions of young people.
It adds that children are increasingly being put directly in harm's way as they are being recruited by armed groups and forces on both sides of the conflict.
Millions of children have been forced from their homes and tens of thousands are living in parks, barns and caves.
"This is a war where women and children are the biggest casualty," chief executive Justin Forsyth said during a visit to Lebanon, where 340,000 Syrians have fled.
Mr Forsyth said he met a Syrian refugee boy, 12, who saw his best friend killed outside a bakery.
"His friend was shot through the heart. But initially, he thought he was joking because there was no blood. They didn't realise he had been killed until they took his shirt off," he said.
The Save the Children report cited new research carried out among refugee children by Bahcesehir University in Turkey, which found that one in three reported having been punched, kicked or shot at.
Two thirds of children surveyed said that they had been separated from members of their families due to the conflict and a third said they had experienced the death of a close friend or family member.
"All these children tell you these stories in a matter of fact way and then you realise that there are layers and layers of emotional trauma there," Mr Forsyth said.
Syria's civil war started with peaceful protests against the rule of President Bashar al-Assad two years ago.