* UN investigators accuse all sides of killings, torture
* Conclude govt forces and militia guilty of Houla massacre
* War crimes by rebels, not of same gravity or scale
* Security Council urged to take action given evidence
By Stephanie Nebehay
GENEVA, Aug 15 (Reuters) - Syrian government forces and allied militia have committed war crimes including murder and torture of civilians in what appears to be state-directed policy, U.N. investigators said on Wednesday.
Syrian rebels fighting to topple President Bashar al-Assad had also committed war crimes, including executions, but on a smaller scale than those by the army and security forces.
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.
"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, one of two commissioners aided by some 20 investigators, told Reuters in a telephone interview.
Paulo Pinheiro, the commissioner who led the probe, said Syria's army of 300,000 had targeted rebel-held areas of cities with heavy artillery and helicopters. It had "much more means to inflict war crimes, for example bombing civilian populations".
"Besides evidence, we have names connected to the evidence," Pinheiro told Reuters, speaking from his native Brazil.
"But we are not a judicial or prosecutorial body. This is a problem for the Security Council, not for us."
The Security Council can refer a case to the prosecutor of the International Criminal Court (ICC), the U.N. war crimes tribunal, but Russia and China - which have veto power - have been loath to condemn Syria.
The independent investigators 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 staff of the Syrian Arab Red Crescent, a war crime, they said.
A Syrian air strike killed 30 people in a rebel-held town on Wednesday, a local doctor said, and a mass kidnapping linked to Syria in neighbouring Lebanon raised the prospect of sectarian violence spreading.
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 provinces, their similar complexity and integrated military/security apparatus "indicate involvement at the highest levels of the armed and security forces and the government".
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.
"We have many instances reported to us where the anti-government forces have executed prisoners. They say they don't have detention facilities and are not based in one territory and can't take care of them. This is a war crime," Pinheiro said.
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.
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 killed in Houla, including some by 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."