Children in Syria have accused troops of using them as human shields, a UN report has revealed.
Some children said they had been forced to ride on tanks to stop attacks by opposition fighters, the report said.
The UN's Special Representative for Children and Armed Conflict said children were being tortured in detention and slaughtered in massacres.
Radhika Coomaraswamy told the BBC that 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.
Ms 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, adding: "Those kinds of things we don't see elsewhere."
The UN's annual report on children and armed conflict cited one attack on the village of Ayn l'Arouz in Idlib province on 9 March 2012.
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.
Despite all international diplomatic efforts, bloodshed is continuing on a daily basis. Activists say the regime of President Bashar al-Assad is deliberately targeting civilian populations, while the government blames the violence on "armed gangs".
The US has expressed fears that the Syrian government "may be organising another massacre" in the town of Haffa in Latakia province, where UN military observers have been denied access.
Earlier this month, activists said Syrian government forces killed 108 people in the region of Houla, in Homs province, and 78 people in the village of Qubair, in Hama province.
UN chief Ban Ki-moon joined Kofi Annan in demanding that UN observers be admitted to the area.
Witnesses reported tanks parked along the edge of the town and said there was a lack of medical facilities to treat the wounded.
Mr Ban also released a statement condemning "intensive military operations" by government forces in the city of Homs as well as firing from helicopters on other towns. He said the actions had caused heavy civilian casualties.
The UK-based Syrian Observatory for Human Rights said more than 100 people were killed in violence across Syria on Monday. The figures cannot be verified.