The UN's Special Representative for Children and Armed Conflict said that children were being tortured in detention and slaughtered in massacres. The report said rebel forces were also using children on the front line.
Separately the United States(US) has accused Syria of planning another massacre, while the UN chief condemned the rising violence. The UN special representative, Radhika Coomaraswamy, told the BBC her team had returned from Syria with "horrific" reports.
She said she had never seen a similar situation where children were not spared - and even targeted - in a conflict.
"Many former soldiers spoke about shooting into civilian areas, seeing children, young children being killed and maimed," she said. "We also had testimonies and saw children who had been tortured, and who carried the torture marks with them. We also heard of children being used - this was recounted to us by some children - of being put on tanks and being used as human shields so that the tanks would not be fired upon."
However, she also criticised the opposition Free Syrian Army for endangering children. "For the first time we heard of children being recruited by the Free Syrian Army mainly in medical and service orientated jobs but still on the front line," she said.
Coomaraswamy said the suffering inflicted on children in Syria was unusual even for combat situations. "We are really quite shocked. Killing and maiming of children in cross-fire is something we come across in many conflicts but this torture of children in detention, children as young as 10, is something quite extraordinary, which we don't really see in other places." She said that in recent massacres children under the age of 10 had been summarily killed.
The UN's annual report on children and armed conflict cited one attack on the village of Ayn l'Arouz in Idlib province on March 9. It quoted a witness saying how several young children were forcibly taken from their homes and "used by soldiers and militia members as human shields, placing them in front of the windows of buses carrying military personnel into the raid on the village". Other children described being beaten, blindfolded, subjected to stress positions, whipped with heavy electrical cables, scarred by cigarette burns and in one case subjected to electrical shock during interrogations, the report said.A team of UN military observers is currently inside Syria as part of a six-point peace plan brokered by UN-Arab League envoy Kofi Annan. (BBC)