A new UN report states that in addition to the widely publicised chemical weapons attack on August 21 near Damascus, such weapons were probably used in four other locations in Syria between March and late August.
The report, published on Thursday, examined seven attacks in which chemical weapons use had been alleged.
"The United Nations Mission concludes that chemical weapons have been used in the ongoing conflict between the parties in the Syrian Arab Republic," noted the report by chief UN investigator Ake Sellstrom.
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The UN Mission investigated seven instances of the possible use of chemical weapons.
The report cited "credible evidence" and "evidence consistent with the probable use of chemical weapons" in Ghouta, Khan al-Assal, Jobar, Saraqeb and Ashrafieh Sahnaya.
However, the UN said it could not corroborate their use in Bahhariyeh and Sheik Maqsoud.
In the rebel-held Damascus suburb of Ghouta, UN inspectors cited "clear and convincing" physical evidence that the deadly nerve agent sarin was used on a large scale against civilians, but this was already public knowledge as Ghouta was the focus of a previous interim report.
Sarin was also probably used in smaller quantities in Jobar, Saraqeb and Ashrafiah Sahnaya, the UN concluded.
In Khan al-Assal, inspectors collected credible information that chemical weapons were used on March 19, while they collected evidence "consistent with probable use of chemical weapons" against soldiers in Jobar on August 24.
The report also found that chemical weapons were probably used on a small scale in Saraqeb and in Ashrafiah Sahnaya.
The UN's investigative team was unable to make on-site visits to almost all of the sites where chemical weapons were allegedly used, mostly because of poor security conditions.
The UN did not determine whether the government or opposition was responsible for the alleged attacks.
Sellstrom delivered his report on Thursday to UN Secretary-General Ban Ki-moon, who urged the elimination of the deadly weapons not only in Syria but everywhere.
Ban said he would address the 193-member General Assembly on Friday and the UN Security Council on Monday about the report's findings.
The Syrian President Bashar al-Assad's government agreed to destroy its chemical weapons arsenal after the August 21 Ghouta attack, which killed hundreds of people and led to threats of US air strikes.
More than two years of civil war in Syria have killed more than 100,000 people.