* Report does not identify who used sarin-filled rockets
* France, Britain, U.S. say U.N. findings point to Assad
* Government, rebels blame each other for Aug. 21 attack
* U.N. chief calls attack "grave crime," seeks justice
By Louis Charbonneau and Michelle Nichols
UNITED NATIONS, Sept 16 (Reuters) - U.N. chemical investigators on Monday confirmed the use of sarin nerve agent in an Aug. 21 poison gas attack outside the Syrian capital in a long-awaited report that confirmed the West's suspicions but was not intended to assign blame.
"This is the most significant confirmed use of chemical weapons against civilians since Saddam Hussein used them in Halabja (Iraq) in 1988," U.N. Secretary-General Ban Ki-moon said. "The international community has pledged to prevent any such horror from recurring, yet it has happened again."
The U.N. team was investigating only whether chemical weapons were used in a deadly assault on the rebel-held Damascus suburb of Ghouta. It was not charged with concluding who launched the attack.
"On the basis of the evidence obtained during the investigation of the Ghouta incident, the conclusion is that chemical weapons have been used in the ongoing conflict between the parties in the Syrian Arab Republic, also against civilians, including children, on a relatively large scale," said the report by chief U.N. investigator Ake Sellstrom of Sweden.
"In particular, the environmental, chemical and medical samples we have collected provide clear and convincing evidence that surface-to-surface rockets containing the nerve agent sarin were used," it said.
The report said the weather conditions on Aug. 21 ensured that as many people as possible were injured or killed. Temperatures were falling between 2 a.m. and 5 a.m., it said, which meant that air was moving downwards toward the ground.
"Chemical weapons use in such meteorological conditions maximizes their potential impact as the heavy gas can stay close to the ground and penetrate into lower levels of buildings and constructions where many people were seeking shelter," it said.
The results of Sellstrom's investigation are not surprising. Several weeks ago U.S. Secretary of State John Kerry announced that sarin had been used in the chemical attack on the Ghouta region. The United States said 1,400 people were killed, including more than 400 children.
"The United Nations Mission has now confirmed, unequivocally and objectively, that chemical weapons have been used in Syria," Ban told the Security Council.
"This was a grave crime and those responsible must be brought to justice as soon as possible," he said.
On Friday, Ban said Syrian President Bashar al-Assad "has committed many crimes against humanity," though he did specifically blame him for the Ghouta attack. He added that Assad would be held to account for his crimes.
RESPONSIBILITY UNCLEAR
Syria and Russia have blamed the Aug. 21 attack on the rebels. The rebels, the United States and other Western powers blame forces loyal to Assad for the Ghouta attack.
It is not immediately clear whether any of the details in the report suggested culpability.
Britain, France and the United States all said the U.N. report left "no doubt" that Assad government was responsible for the chemical attack.
The investigators studied five impact sites and were able to determine the likely trajectory of the projectiles at two sites - Moadamiyah and Ein Tarma.
Eliot Higgins, who blogs under the name of Brown Moses and has been tracking videos of weapons used in the Syria conflict, wrote that he has not seen the opposition using the munitions identified in the report: a variant of the M14 artillery rocket and a 330 mm caliber artillery rocket.
Rebels have seized all kinds of weapons from military depots across the country in the 2 1/2-year civil war.
But Amy Smithson, a chemical weapons expert at Monterey Institute, said the Aug. 21 attack bore "so many hallmarks of a military trained in chemical warfare doctrine" and not an untrained force.
"Multiple sites, simultaneously targeted. The early morning hours of the attack are when winds are at their lowest and temperatures at their coolest - the very conditions conducive to having toxic gas stay on the target," she told Reuters.
"The Assad government has been in the business of chemical weapons since the 1970s. They are trained in military doctrine. They also have chemical delivery systems that the rebels don't," she said.
Ban told the council that 85 percent of the blood samples taken by investigators tested positive for sarin and almost all the biomedical samples tested positive for exposure to sarin.
The U.N. confirmation of sarin gas use on Aug. 21 comes as France, Britain and the United States agreed in Paris to seek a "strong and robust" U.N. resolution that sets binding deadlines on removal of chemical weapons.
Those talks followed a weekend deal on Syria's chemical weapons reached by the United States and Russia that could avert U.S. military action.
Ban urged the Security Council consider ways to ensure enforcement and compliance with the U.S.-Russia plan. "I agree there should be consequences for non-compliance. Any use of chemical weapons by anyone, anywhere, is a crime," he said.
The United Nations says more than 100,000 people have died in Syria since March 2011.
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