Millions of Syrian children could be living in a state of "toxic stress" due to prolonged exposure to the horrors of war, aid group Save the Children says.
The damage to an entire generation of children could soon become irreversible without immediate help, it adds.
The stress of war has led to increased bedwetting, self-harm, suicide attempts and aggressive behaviour among many children, according to a new report.
The findings are based on hundreds of interviews in Syria.
Save the Children says its study is the largest of its kind into the mental health and well-being of Syria's children amid the war, which began in 2011 and has left more than 300,000 people dead.
The report, Invisible Wounds, reveals a "terrifying mental health crisis among children trapped in Syria".
Save the Children spoke to more than 450 people in seven of Syria 14 governorates as part of its study, including children of varying ages, parents, caregivers, social workers, aid workers and teachers.
Almost all children and 84% of adults said that bombing and shelling was the number one cause of psychological stress for children
Two-thirds of children have either lost a loved one, had their house bombed or shelled, or been injured as a result of the war, according to adults interviewed (some had suffered more than one of these traumatic events)
71% of interviewees said that children were increasingly suffering from frequent bedwetting and "involuntary urination" - symptoms of toxic stress and post-traumatic stress disorder (PTSD)
48% of adults said they had seen children who had lost their ability to speak or begun to suffer from speech impediments since the war began
Nearly half of those interviewed said children "regularly or always have feelings of grief or extreme sadness"