Syrian rebels fighting to topple President Bashar al-Assad have also committed war crimes, including executions, but these “did not reach the gravity, frequency and scale” of those carried out by the army and security forces, they said.

The report called for the U.N. Security Council to take “appropriate action” given the gravity of documented violations by all sides in a 17-month conflict that investigators said had become a civil war.

The Security Council can refer a case to the prosecutor of the International Criminal Court (ICC), the United Nations war crimes tribunal, but Russia and China - which have veto power - have been loathe to condemn Syria.

“We have identified both parties as guilty of war crimes and of course a greater number and of bigger variety from the government side,” Karen AbuZayd, a senior investigator and former head of the U.N. agency helping Palestinian refugees, told Reuters in a telephone interview from the United States.

“What happened on the government side appears to be a policy of the state. It is not just widespread but similar large-scale complex operations, how they are carried out, the way the military and security work together,” she said.

The independent investigators, led by Brazilian Paulo Pinheiro, conducted more than 1,000 interviews, mainly with Syrian refugees or defectors who have fled to neighbouring countries, over the past year to produce their latest 102-page report to the U.N. Human Rights Council.

They found “reasonable grounds” to affirm that government forces and their allied shabbiha militia had committed crimes against humanity, war crimes and other gross violations.

These included “unlawful killing, torture, arbitrary arrest and detention, sexual violence, indiscriminate attack, pillaging and destruction of property”.

Government forces and shabbiha militia had raped men, women and children in acts that could be prosecuted as crimes against humanity, the investigators said. Government troops had targeted aid workers of the Syrian Arab Red Crescent, a war crime, they said.

A Syrian air raid killed at least 30 people in the rebel-held northern border town of Azaz on Wednesday, a local doctor said, and a bomb went off near U.N. and military sites in the capital Damascus, wounding three.

“PURSUANT TO STATE POLICY”

Evidence confirmed a previous finding that “violations had been committed pursuant to State policy,” the U.N. report said.

“Large-scale operations conducted in different governorates (provinces), their similar modus operandi, their complexity and integrated military/security apparatus indicate involvement at the highest levels of the armed and security forces and the government,” it said.

Rebels had killed captured government soldiers, shabbiha and suspected informers, sometimes after summary trials, the investigators said. “Executing a prisoner without affording fundamental judicial guarantees is a war crime,” they added.

Both government forces and armed insurgents had displayed “more brutal tactics and new military capabilities” as fighting escalated during recent months, the report said.

Each side had violated children’s rights, it said. At least 125 youths under age 18, mainly boys, had been killed since February, while others were arbitrarily arrested without charge.

“Children described having been beaten, whipped with electrical cables, burned with cigarettes and subjected to electrical shocks to the genitals,” the investigators said of those in the custody of state forces.

Armed insurgents continue to use children as couriers or to help with medical evacuations, they said, also voicing concern at reports that rebels have deployed some minors as fighters.

Completing their probe into a massacre in the town of Houla in May - which the government blamed on Islamist “terrorists” - they said government forces and shabbiha fighters were responsible for the killings of more than 100 civilians.

Forty-one children were among the Houla victims, including some who died during shelling, “but most appeared to have been shot at close range”.

The investigators said they would update their confidential list of suspects or units responsible for crimes and give it to U.N. rights boss Navi Pillay when their mandate ends next month.

“We don’t name any names in the report,” AbuZayd said. “But the evidence is recorded all along.”