Children in Syria have been tortured, maimed and sexually abused by Bashar al-Assad's forces and recruited for combat by the rebels fighting to topple him during the country's conflict of almost three years, a new UN report says.

The report, which highlights the treatment of children in the conflict from the beginning of the uprising against the president in March 2011 until 15 November 2013, was released this week to the security council and posted on the UN website on Tuesday.

The report cites the UN secretary-general, Ban Ki-moon, saying that Syrian children were subjected to "unspeakable suffering" during that time. Ban urged Syria's warring sides to "take, without delay, all measures to protect and uphold the rights of all children in Syria".

The uprising against Assad's rule began with largely peaceful protests in 2011 but evolved into a bloody civil war that has killed more than 130,000 people, according to activists. Millions of Syrians have been driven from their homes, seeking shelter in neighbouring countries or in safer parts of their homeland.

The conflict has hit the country's children hard. The UN said government forces had been responsible for the arrest, arbitrary detention, ill treatment and torture of children. Children as young as 11 have been detained by the authorities on suspicion of having links with armed groups.

Children in government custody have reportedly been beaten with metal cables, whips and wooden and metal batons, and suffered electric shock, mock executions, cigarette burns, sleep deprivation and solitary confinement, and sexual violence, including rape and threats of rape, the report says.

It was not clear what methodology for the findings was used and the summary of the report posted on the UN website did not say how investigators obtained their information.

Allegations of sexual violence by opposition groups were also received, but the UN was unable to further investigate them due to lack of access to areas under rebel control, the report says.

While Assad's forces have used children as human shields in the fighting, the report also condemned rebels for "recruitment and use of children both in combat and support roles, as well as for conducting military operations".

During the first two years of the conflict most killings and maimings of children were attributed to government forces, the UN report says. During 2013 the opposition forces increasingly "engaged in such acts", it adds. "Armed opposition groups also engaged in the summary execution of children."

The report states that UN investigators have not been able to reach many of the rebel-held areas for lack of security there, and consequently have been unable to further investigate and document those violations.