Chemical weapons were "probably used" at five out of seven sites in Syria, UN investigators say in a report.
In two cases, the weapons affected soldiers, and in a third, soldiers and civilians, the report says.
The study has not determined whether the government or rebels had used the weapons.
The inspectors had already confirmed that the deadly nerve gas sarin was used in an attack near Damascus on 21 August which killed hundreds.
The report said it had proof that chemical weapons were used in Ghouta, Khan al-Assal, Jobar, Saraqueb and Ashrafieh Sahnaya.
It cites "credible evidence" or "evidence consistent with the probable use of chemical weapons", but says it still lacks specific information about the chain of events at many of the sites.
The inspectors said they could not corroborate the use of chemical weapons in incidents in Bahariyeh and Sheik Maqsood.
Their findings were based on assessment of evidence found at the various sites, including surface-to-surface rockets, medical samples, and on interviews with victims and medical clinicians.
The report says the team was unable to make on-site visits to almost all of the sites where chemical weapons allegedly were used, mostly because of poor security conditions.
It also says "the significant time delay" between the alleged events and the investigation affected their scientific testing.
"The United Nations Mission remains deeply concerned that chemical weapons were used in the ongoing conflict between the parties in the Syrian Arabic Republic, which has added yet another dimension to the continued suffering of the Syrian people," the report says.
On Friday the UN Secretary General Ban Ki-moon is due to brief the UN's General Assembly on the inspectors' findings in a closed session.
In the aftermath of the attack on the Ghouta region near Damascus, Syrian President Bashar al-Assad allowed international inspectors to begin destroying chemical weapons under the Syrian government's control on the basis of a US-Russian agreement.
The Organisation for the Prohibition of Chemical Weapons (OPCW) confirmed last month that some of the arms will be "neutralised" aboard a US Navy ship. The operations on board the ship should be completed by the end of the year, the OPCW says.
However on Monday the head of the OPCW, Aake Sellstrom, warned that it would be "quite difficult to meet" the 31 December deadline for transporting the most toxic chemicals out of the country.
The remit of the report was not to apportion blame on either side of the conflict.
More than 100,000 people have been killed in the fighting that has ravaged Syria for two-and-a-half years, according to the UN.
A further two million people have fled Syria and some 4.5 million have been displaced internally.