A report by the Organization for the Prohibition of Chemical Weapons (OPCW) has suggested Syrian civilians could have been exposed to Sarin gas or similar toxic chemicals.
OPCW is currently investigating 11 incidents of the use of toxic chemicals indicated by the Syrian Government.
OPCW chief Ahmet Uzumcu said: "Further investigation would be necessary to determine when or under what circumstances such exposure might have occurred."
The UN Security Council is set to discuss Syria's chemical weapons exploit this week.
The OPCW fact-finding mission determined mustard gas and chlorine had been used in the thickly populated civilian localities.
According to Amnesty International, hundreds of civilians have been killed in chemical weapons attacks since the start of the Syrian crisis four years ago.
International humanitarian law prohibits the use of chemical weapons.
In 2013, Syria agreed to destroy its chemical weapons programme after hundreds of people were killed in a Sarin gas attack in the capital city of Damascus.
In August, the UN Security Council (UNSC) approved a draft resolution for setting up a mechanism to identify those responsible for the use of chemical weapons in Syria.
Unanimously adopted by the 15-member body, the resolution stresses the obligation of the Syrian Government and all parties in the country to extend full cooperation to the UN and OPCW for the mission.
On 4 January, OPCW successfully disposed of 75 cylinders of hydrogen fluoride at its facility in Texas.