According to leading United Nations investigator Carla del Ponte, testimony from victims of the Syrian conflict suggests rebels have used the nerve agent sarin.
Carla del Ponte told Swiss TV there were “strong, concrete suspicions but not yet incontrovertible proof”.
However, she said her panel had not yet seen evidence of government forces using chemical weapons.
Syria has recently come under growing Western pressure over the alleged use of such weapons.
Carla del Ponte, who serves on the UN Independent International Commission of Inquiry on Syria, said in an interview with Swiss-Italian TV: “Our investigators have been in neighboring countries interviewing victims, doctors and field hospitals.
“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.”
Carla del Ponte, a former Swiss attorney-general and prosecutor with the International Criminal Tribunal for the former Yugoslavia, did not rule out the possibility that government troops might also have used chemical weapons, but said further investigation was needed.
She gave no details of when or where sarin may have been used.
Her commission was established in August 2011 to examine alleged violations of human rights in the Syrian conflict since March 2011 and is due to issue its latest report to the UN Human Rights Council in Geneva in June.
A separate United Nations team was established to look specifically into the issue of chemical weapons.
The team is ready to go to Syria but wants unconditional access with the right to inquire into all credible allegations.
Both the Syrian government and the rebels have in the past accused each other using chemical weapons.
Sarin, a colorless, odorless gas which can cause respiratory arrest and death, is classed as a weapon of mass destruction and is banned under international law.