Syrian rebels have used the deadly nerve agent sarin in their fight against President Bashar al-Assad's regime, according to testimony from victims, UN human rights investigator Carla del Ponte said.
"According to the testimonies we have gathered, the rebels have used chemical weapons, making use of sarin gas," del Ponte, a former war crimes prosecutor, said in an interview with Swiss radio late on Sunday.
She said there was "still not irrefutable proof, (but) very strong suspicions, concrete suspicions that sarin gas has been used. Assistance to victims shows this."
Her comments follow Israeli air strikes on military sites near Damascus on Sunday and come amid suspicions that Assad's regime has used chemical weapons in the 16-month conflict.
Del Ponte said the UN commission of inquiry on Syria, which she is a member of, was far from finishing its probe.
"We still have to deepen our investigation, verify and confirm (the findings) through new witness testimony, but according to what we have established so far, it is at the moment opponents of the regime who are using sarin gas," she said.
Del Ponte also said the commission might still find proof that the Syrian regime was also using this type of chemical weapon.
US President Barack Obama has said that the use of chemical weapons in the Syria conflict was a "red line" for his administration but has said he does not foresee US troops on the ground in Syria.
Set up two years ago at the behest of the UN Human Rights Council, the commission has so far been unable to gain access to Syria as Damascus has ignored repeated requests for entry.
Instead, it has interviewed over 1,500 refugees and exiles as a basis for its reports and its charges that both the government forces and their allies and opposition forces of carrying out war crimes in Syria, where more than 70,000 people have been killed since the violence exploded in March 2011.
Sarin is a powerful neurotoxin developed by Nazi scientists in the 1930s.
Originally developed as a pesticide, sarin was used to deadly effect in the 1988 raid on the Kurdish village of Halabja in northern Iraq. A Japanese cult also used sarin in two attacks in the 1990s.
The gas works by being inhaled or absorbed through the skin and kills by crippling the nervous system.
Symptoms include nausea and violent headaches, blurred or tunnel vision, drooling, muscular convulsions, respiratory arrest, loss of consciousness and then death, according to the US Centres for Disease Control and Prevention.
In high doses, sarin paralyses the muscles around the lungs and prevents chemicals from "switching off" the body's secretions, so victims suffocate or drown as their lungs fill with mucus and saliva.
Even a tiny dose of sarin -- which, like other nerve gases such as soman, tabun and VX, is odourless, colourless and tasteless -- can be deadly if it enters the respiratory system, or if a drop comes into contact with the skin.