Syrian rebels have used the deadly nerve agent sarin in their fight against President Bashar al-Assad's regime, according to evidence from victims and doctors, 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 and a member of a UN commission of inquiry on Syria, said in an interview with Swiss Italian broadcaster RSI 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 come amid growing Western suspicions that Assad's regime has used chemical weapons in the 26-month conflict and follow Israeli raids on military sites near Damascus over the weekend.
She said that as far as the UN commission had been able to determine, "sarin gas has been used ... by opponents, by rebels, not by government authorities."
"This is not surprising since the opponents have been infiltrated by foreign fighters," she said.
But she said the investigation was far from over, adding: "We have not excluded the use of chemical weapons by the government."
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 pressed for further evidence before taking action.
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.
The commission is set to publish its next report on the situation in Syria at the end of May and will present its findings to the Human Rights Council during its next session in June.
Del Ponte reiterated in Sunday's interview her call for the deadlocked UN Security Council to refer the Syrian violence to the International Criminal Court in the Hague.
"If no-one gives a court the responsibility of investigating these crimes, then our work will be in vain," she said.
Sarin is a powerful neurotoxin developed by Nazi scientists in the 1930s.
Originally developed as a pesticide, sarin was used to deadly effect in air raids in 1988 by former Iraqi president Saddam Hussein's forces on the Kurdish village of Halabja in northern Iraq that left an estimated 5,000 people dead, mostly women and children. It is regarded as the worst ever gas attack targeting civilians.
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.
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.