Syrian regime forces attacked the village of Qmenas with chemical weapons in March 2015, UN experts said in a report released on Friday.
But they were unable to determine who was responsible for two other chemical weapons attacks – against Binnish in Idlib Province in March 2015 and Kafr Zita in Hama Province in April 2014.
The report was presented on Friday to the UN Security Council.
The UN-led joint investigative mechanism (JIM) in late August reported that Syrian regime forces had carried out at least two chemical attacks in 2014 and 2015 and the Daesh terrorist organisation had used mustard gas as a weapon.
Of the nine total alleged chemical attacks it is considering in its ongoing probe, the JIM has now attributed three to the Syrian regime and one to Daesh.
In its fourth report investigators concluded that there is now "sufficient information" that the attack on Qmenas "was caused by a Syrian Arab Armed Forces helicopter dropping a device from a high altitude which hit the ground and released the toxic substance that affected the population."
Investigators say the substance may have been chlorine gas, based on the symptoms the victims displayed.
In Kafr Zita, however, the JIM could not confirm that Syrian regime forces had used barrel bombs to dump toxic substances because "the remnants of the device allegedly used had been removed," the report said.
Investigators also said that a "canister with traces of chlorine" was found in Binnish, though the container could not be "linked to any of several incident locations identified."
The inquiry's mandate was extended until October 31 to finish the probe.
Governments in Paris, London and Washington have already called for sanctions against perpetrators of chemical attacks in Syria, including against the regime in Damascus.
But the regime has been shielded by its ally Russia, which has questioned the JIM findings and said the evidence is not conclusive enough to warrant sanctions.
The Syrian regime agreed to get rid of its chemical stockpile and to refrain from making any use of toxic substances in warfare when it joined the Chemical Weapons Convention in 2013, under pressure from Russia.