Syrian rebels fighting to overturn the regime of President Bashar Al Assad have used the nerve gas sarin, according to UN investigators.
Carla del Ponte, who previously acted as prosecutor against war crimes in Yugoslavia, said evidence pointed to the use of Sarin by rebel troops, and not government sources as was previously hinted.
"There are strong, concrete suspicions, but not yet incontrovertible proof, of the use of sarin gas," said Del Ponte, a member of a panel investigating allegations of war crimes in Syria.
"Our investigators have been in neighbouring countries interviewing victims, doctors, and field hospitals. We collected witness testimony that made to appear that some chemical weapons were used, in particular nerving gas.
"It appears to our investigation that was used by the opponents, by the rebels. This was used on the part of the opposition, the rebels, not by the government authorities."
Her findings come in direct contradiction to US claims that the regime was to blame for the use of the sarin gas.
The inquiry gave no details as to when or where the gas may have been used.
Del Ponte's inquiry is separate from the main UN probe into chemical weapons use in Syria.
Each side accused the other of using chemical weapons in the fighting for Aleppo in March. Britain and France say they have evidence that nerve gas was used.
Israeli airstrikes have killed at least 42 government soldiers in recent days, according to the Syrian Observatory for Human Rights.
Israel has denied it is supporting anti-regime rebels, as the Assad regime claims, saying it is seeking to block the transfer of weapons to Hezbollah militias into neighbouring Lebanon.