Syrian children have been raped, tortured and used in combat as human shields, the United Nations said in a damming report detailing the horrors of the on-going civil war.

Covering the period from March 2011 to November 2013, the report found children as young as 11 have been subjected to beatings with metal cables, whips and batons, ripping out of fingernails and electric shocks, including to the genitals. In the most brutal cases, some of them witnessed the torture of relatives and mock executions.

Sexual violence, including rape or the threat of rape, was reportedly used to "humiliate, harm and force confessions".

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The report cited the case of a 16-year old who was arrested with five other boys aged 13 to 15 at a checkpoint near his school and detained in a centre run by intelligence forces. The teenager allegedly witnessed his 14-year old male friend being sexually assaulted and then killed.

The UN said it received allegations of sexual violence by armed opposition groups but was unable to corroborate them given the "lack of access."

UN chief Ban Ki-moon said:"The suffering endured by the children in the Syrian Arab Republic since the outset of the conflict, as documented in this report, is unspeakable and unacceptable."

The UN accused both sides of the conflict of committing "grave violations" against children. Government troops loyal to Bashar al-Assad were largely responsible for the attacks in the early stages of war, it said, but as the conflict intensified, rebels "became more organised" and carried out similar assaults on children.

Known perpetrators of attacks on children include the Syrian Armed Forces; Syrian intelligence forces, such as the Syrian Military Intelligence, Air Force Intelligence, the National Security Bureau, the Political Security Directorate and the General Intelligence Directorate, the report said.

While the UN could not confirm the recruitment of children by government forces, it said Assad troops were responsible for the arrest and arbitrary detentions of children with some of them being used as human shields in combat. Opposition groups have both recruited and used children in support roles and in combat, the report said.

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"Of particular concern were cases of recruitment or attempted recruitment of children within refugee populations in neighboring countries, " Ban added. " The majority of incidents were related to recruitment by Free Syrian Army-affiliated groups or Syrian Kurdish armed groups."

Since March 2011, more than 100,000 people have been killed in Syria, including more than 10,000 children, according to the United Nations.

Full report here