(Adds quote from Ban, detail)
By Louis Charbonneau and Michelle Nichols
UNITED NATIONS, Dec 12 (Reuters) - Chemical weapons were likely used in five out of seven attacks investigated by U.N. experts in Syria, where a 2 1/2-year civil war has killed more than 100,000 people, according to the final report of a U.N. inquiry published on Thursday.
U.N. investigators said the deadly nerve agent sarin was likely used in four incidents, in one case on a large scale.
The report noted that in several cases the victims included government soldiers and civilians, though it was not always possible to establish with certainty any direct links between the attacks, the victims and the alleged sites of the incidents.
"The United Nations Mission concludes that chemical weapons have been used in the ongoing conflict between the parties in the Syrian Arab Republic," the final report by chief U.N. investigator Ake Sellstrom said.
Syria's U.N. Ambassador Bashar Ja'afari and the opposition Syrian National Coalition did immediately comment on the 82-page report.
The investigation found likely use of chemical weapons in Khan al-Assal, near the northern city of Aleppo, in March; in Saraqeb, near the northern city of Idlib, in April; and in Jobar and Ashrafiat Sahnaya, near Damascus, in August.
As initially reported by Sellstrom in September, there was "clear and convincing" evidence that sarin was used on a large-scale against civilians in the rebel-held Damascus suburb of Ghouta on Aug. 21, killing hundreds of people.
In the final report on Thursday, the experts said sarin had likely also been used on a small-scale in Jobar, Saraqeb and Ashrafiat Sahnaya.
The inquiry was only looking at whether chemical weapons were used, not who used them. The Syrian government and the opposition have accused each other of using chemical weapons, and both have denied it.
TOTAL 16 ALLEGATIONS
Rebels have seized all kinds of weapons from military depots across Syria, according to the United Nations. Western powers say the rebels do not have access to chemical arms.
U.N. Secretary-General Ban Ki-moon established the Sellstrom investigation after the Syrian government wrote to Ban accusing the rebels of carrying out the chemical weapons attack in Khan al-Assal.
Sellstrom delivered the final report to Ban on Thursday. Ban will brief the U.N. General Assembly on the report on Friday and the U.N. Security Council on Monday.
"The use of chemical weapons is a grave violation of international law and an affront to our shared humanity," Ban said. "We need to remain vigilant to ensure that these awful weapons are eliminated, not only in Syria, but everywhere."
The United Nations has now received 16 reports of possible chemical weapons use in Syria, mainly from the Syrian government, Britain, France and the United States. The experts looked closely at seven of those cases.
The U.N. experts were from the Organization for the Prohibition of Chemical Weapons and the World Health Organization.
France, Britain and the United States said the technical details of Sellstrom's initial September report on the Aug. 21 attack pointed to government culpability, while Syria and Russia blamed the rebels.
Syrian President Bashar al-Assad's government agreed to destroy its chemical weapons arsenal after the Aug. 21 Ghouta attack, which had led to threats of U.S. air strikes. Syria also acceded to the Chemical Weapons Convention.
The U.N. Security Council adopted a resolution in September to enforce the deal, brokered by the United States and Russia, which requires Syria to account fully for its chemical weapons and for the arsenal to be removed and destroyed by mid-2014.
The Hague-based Organization for the Prohibition of Chemical Weapons has been charged with supervising the elimination of Syria's chemical arsenal. (Editing by Sandra Maler, Jim Loney and Mohammad Zargham)