Syria's children are "trapped in a living nightmare", according to UNICEF's regional director.

Addressing the UN Security Council, Geert Cappelaere said: "Indeed, it is difficult to imagine what words could still adequately convey the unspeakable horrors endured by Syria's children every day. Tens of thousands of children have already been killed. Millions have been uprooted, some more than once. Too many have been deprived of basic medical care and safe drinking water. Too many have witnessed the death of their loved ones and the destruction of the places they once thought are safe: their homes, their schools, their playgrounds.

"We must ask ourselves: what is left of childhood for the boys and girls of Syria?"

He drew a horrifying picture of the hardships endured by the children of Aleppo. "Children wonder why this is happening. Children are wondering why no one is helping to stop this," he said.

He continued: "Amir, a five-year-old boy, is one of those children. Our team met him in Aleppo very recently. He and his sister were playing outside when their house was shelled. His sister was injured in the face and eye, while Amir sustained severe burns all over his body. He had to undergo two painful surgeries to replace the burned skin in one of the very few health facilities remaining in Aleppo."

Amir, Cappelaere said, was "lucky". But he also spoke of a father "living with the trauma and deep regret of simply letting his eight and 10 year old daughters go to school. They left their makeshift home one morning with their schoolbags on their backs. Only their lifeless bodies returned after a shell slammed into their classroom. UNICEF colleagues could barely look into the eyes of the father suffering so much pain."

He said eastern Aleppo's health system was crumbling and that children were sometimes just left to die because of limited resources. "The violence must stop. Nothing justifies the indiscriminate slaughter of civilians, especially children. Parties to the conflict must protect civilians – not least children – and allow immediate access for humanitarian agencies to provide assistance."

Cappelaere said an estimated six million children inside Syria needed help, more than two million of them in hard-to-reach areas. There were nearly half a million living under siege. UNICEF and other agencies were prevented from bringing them help due to lack of access.

He painted a grim picture of schools destroyed and education halted, lack of water, malnutrition and poor hygiene – only a third of Syria's sewage is now treated. Children had been killed, maimed and abducted for recruitment as child soldiers.

He said: "The protection of children should be, at all times, a primary consideration for all of us. We have failed them over the last six years, and we continue to fail the children in Syria. This is not only jeopardising children's lives, but the future of the country, the future of the region and the future of the whole world."

He concluded: "Until the guns are silenced, and remain silent, children in Syria will continue to ask us, ask you, why."