The UN has gathered testimony that Syrian rebels have used the nerve agent sarin, the BBC reported.
The United Nations independent commission of inquiry on Syria has evidence from victims and medical staff in Syria that the gas has been used by rebel forces. The commission has no evidence that governmnet forces have used chemical weapons, commission member Carla del Ponte said on Sunday.
"Our investigators have been in neighbouring countries interviewing victims, doctors and field hospitals and, according to their report of last week which I have seen, there are strong, concrete suspicions but not yet incontrovertible proof of the use of sarin gas, from the way the victims were treated," she said in an interview with Swiss-Italian television on Sunday.
"This was use on the part of the opposition, the rebels, not by the government authorities," she added.
Del Ponte added that the UN would require further evidence to verify the testimony they've received thus far.
The commission was established in August 2011 to investigate human rights abuses in Syria, whilst a separate panel was established to deal specifically with the issue of chemical weapons.
Both sides have accused the other of using chemical weapons during the conflict, although there has been little evidence to back up the claims.
The US, UK and France have said they had seen evidence that government forces had used sarin gas.
Chemical weapons use is prohibited under international law and the US administration has stated that the use of such weapons would be a "game changer" in the conflict.