THE Syrian government and Islamic State insurgents are both committing war crimes and crimes against humanity in their increasingly brutal fight against each other, UN investigators have said.

Islamic State forces in northern Syria are waging a campaign to instil fear, including amputations, public executions and whippings, they said, while government forces have dropped barrel bombs on civilian areas, including some believed to contain the chemical agent chlorine in eight incidents.

The Damascus regime is also accused of killings, torture and other war crimes that should be prosecuted, the UN said in a report.

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Deaths in custody in Syrian jails are increasing and forensic analysis of 26,948 photographs allegedly taken from 2011-2013 in government detention centres back its "long-standing findings of systematic torture and deaths of detainees".

The report, issued in Geneva, added: "Forced truces, a mark of the government's strategy of siege and bombardment, are often followed by mass arrests of men of fighting age, many of whom disappear."

The UN report, the commission of inquiry's eighth since being set up three years ago, is based on 480 interviews and documentary evidence gathered by its team, which is trying to build a case for future criminal prosecution.

Islamic forces, which are also sweeping through neighbouring Iraq in their attempt to establish a cross-border caliphate, have drawn more experienced and ideologically motivated foreign fighters and established control over large areas in northern and eastern Syria, particularly oil-rich Deir al-Zor, it said.

The report said: "Executions in public spaces have become a common spectacle on Fridays in al Raqqa and ISIS-controlled areas of Aleppo.

"Children have been present at the executions, which take the form of beheading or shooting in the head at close range. Bodies are placed on public display, often on crucifixes, for up to three days, serving as a warning to locals."

Paulo Pinheiro, chairman of the UN panel, said he was also concerned about boys forced to join Islamic State training camps, telling reporters the US should take their presence into account before launching air strikes.

He added: "Among the most disturbing findings in this report are accounts of large training camps where children, mostly boys from the age of 14, are recruited and trained to fight in the ranks of ISIS along with adults."

The US air force has already hit the same group across the border in Iraq and President Barack Obama has called for surveillance flights to gather intelligence on Islamic State units in Syria should he decide to order air strikes there.

The UN report said Islamic State forces had committed torture, murder, acts tantamount to enforced disappearance and forced displacement as part of attacks on civilians in Aleppo and al-Raqqa provinces, amounting to crimes against humanity.

Mr Pinheiro said: "ISIS poses a clear and present danger to civilians, and particularly minorities, under its control in Syria and in the region."

The investigators, who include former UN crimes prosecutor Carla del Ponte, have drawn up four confidential lists of suspects whom they believe should face inter­national justice.

The report reiterates their call for the UN Security Council to refer violations to the International Criminal Court.