The rebel Free Syrian Army (FSA) has rejected claims by UN investigators that it is likely to have used chemical weapons.
Carla Del Ponte, a leading UN human rights investigator, said on Sunday that a UN commission of inquiry had gathered testimony from casualties of the civil war and medical staff indicating that rebel forces had used the nerve agent sarin.
Speaking to Al Jazeera on Monday, Saleem Edris, FSA chief of staff, said he considered the remarks a "huge injustice" and "provocation" to the Syrian people.
Del Ponte, a member of the UN Independent International Commission of Inquiry on Syria, also said that the inquiry had not yet seen evidence of government forces having used chemical weapons, which are banned under international law.
"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.
"This was use on the part of the opposition, the rebels, not by the government authorities," she added.
Del Ponte, a former Swiss attorney-general who also served as prosecutor of the International Criminal Tribunal for the former Yugoslavia, gave no details as to when or where sarin may have been used.
Later on Monday, the Syrian National Coalition (SNC) announced it would hold a symposium on the use of chemical weapons in Syria on Tuesday in Istanbul, with a lawyer and doctor who are members of the group.
Adnan Sillo, a defector from the Syrian military who had previously headed a chemical warfare unit, will also be speaking.
US President Barack Obama has said that the use of chemical weapons in the Syria conflict is 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 more than 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 have carried out war crimes in Syria, where more than 70,000 people have been killed since the uprising began in March 2011.
Sarin is a powerful neurotoxin developed by Nazi scientists in the 1930s.
A Japanese cult also used sarin in two attacks in the 1990s.Originally developed as a pesticide, it was used to deadly effect in the 1988 raid on the Kurdish village of Halabja in northern Iraq.
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.