* Both sides perpetrating war crimes, U.N. investigators say
* Murders, torture, hostage-taking, shelling with impunity
* Urge Security Council to refer perpetrators to int'l justice
* Eight massacres attributed to Syrian govt forces, one to rebels (adds details, quotes)
By Stephanie Nebehay
GENEVA, Sept 11 (Reuters) - U.N. rights investigators have established that Syrian government forces were almost certainly responsible for two massacres last May in which up to 450 civilians were killed, a report published on Wednesday said.
The report documented eight mass killings in all, attributing all but one to government forces, but said both government and rebel fighters had committed war crimes including murder, hostage-taking and shelling of civilians in their battle for territory.
The killings in Baida and Ras al-Nabaa, two pockets of rebel sympathisers surrounded by villages loyal to President Bashar al-Assad on the outskirts of the coastal town of Banias, sent a chilling message of the price to be paid for backing the rebels.
The U.N. commission of inquiry has not been allowed into Syria, but its 20 investigators carried out 258 interviews with refugees, defectors and others, in the region and in Geneva, including via Skype, for their 11th report in two years.
In Baida, it said between 150 and 250 civilians had allegedly been killed, including 30 women, apparently executed, who were found in one house. It said armed rebels were not active in the area at the time.
"Testimonies were consistent that members of the National Defence Forces were actively involved in the raids and in many cases leading them," the report said.
"Accordingly, there are reasonable grounds to believe that government forces and affiliated militias including the National Defence Forces are the perpetrators of the al-Bayda (Baida) massacre."
The next day, as word spread that militia fighters were advancing with army support, hundreds of civilians tried to flee the neighbouring village of Ras al-Nabaa, but were pushed back at checkpoints. Government forces proceeded to shell the village and then militia fighters moved in.
"As they raided the village, civilians were captured and executed," the report said, adding: "The operation did not occur in the context of a military confrontation. Government forces were in full control of the area."
It gave a figure of 150-200 dead in Ras al-Nabaa.
At least 100,000 people have been killed and millions driven from their homes in the conflict, which began in March 2011 as an uprising against Assad and descended into a civil war where mostly Sunni Muslim rebels are pitted against Assad's forces, who are backed by Shi'ite Muslim Iran and Hezbollah.
The only deliberate slaughter of civilians attributed to rebel forces in the period of investigation was in June, when rebels captured Hatla in the eastern province of Deir al-Zor.
"During the takeover, anti-government armed group fighters conducted home invasions, killing and summarily executing (by shooting at close range) many Shia including at least 30 civilians, among them children, women and elderly," it said.
The 42-page report, which largely covered incidents between May and July, also accused forces loyal to President Bashar al-Assad of bombing schools and hospitals, and the rebels of carrying out summary executions after sentencing by Islamic Sharia courts lacking due process.
"The perpetrators of these violations and crimes, on all sides, act in defiance of international law," said the report.
The commission, led by Paulo Pinheiro of Brazil, urged the U.N. Security Council to hold perpetrators accountable for war crimes and crimes against humanity.
The investigators, who include former U.N. war crimes prosecutor Carla del Ponte, analysed photographs, video and satellite imagery, as well as forensic and medical records, to draw up their report.
The team also verified the killing of 450 people during an offensive by Syrian government forces and Lebanese Hezbollah fighters to retake the strategic town of Qusair. Food and water were cut off during a siege, and the town was heavily shelled.
"Approximately half were civilian casualties, killed primarily in the shelling and aerial bombardment of the town in the early days of the offensive," the report said.
U.S. President Barack Obama asked Congress on Tuesday to delay a vote on authorising a military strike against Syria in retaliation for its use of chemical weapons, to give time to a Russian plan to take away Syria's chemical weapons.
The experts said they had received allegations of chemical weapons use, "predominantly by government forces", but could not give details of the incidents, some of which have been investigated by U.N. weapons inspectors in the last few weeks. (Reporting by Stephanie Nebehay, editing by Tom Miles and Mark Heinrich)