Investigator Carla Del Ponte caught UN officials by surprise on Sunday when she said the commission had gathered testimony from casualties and medical staff indicating that rebel forces had used the banned nerve agent sarin.
"The independent international Commission of Inquiry on the Syrian Arab Republic wishes to clarify that it has not reached conclusive findings as to the use of chemical weapons in Syria by any parties to the conflict," it said in a statement.
President Bashar al-Assad's government and the rebels whom his forces have been fighting for more than two years accuse each another of carrying out three chemical weapon attacks in March and December, in Aleppo, Damascus and Homs.
U.S. President Barack Obama has warned any confirmed use of chemical weapons by Assad would cross a "red line" and British Prime Minister David Cameron said late last month there was limited but growing evidence the banned arms had been used.
Bolstering that evidence, a diplomatic source told Reuters on Monday soil samples from Syria have tested positive at Britain's Porton Down military facility for sarin - though it was not clear where the samples came from.
A Syrian medic working near the Turkish border said his patients also showed signs of exposure to the gas - but only a diluted form, possibly released to scare but not kill them.
UN investigator Del Ponte, a former Swiss attorney general who 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. She was speaking in an interview with Swiss-Italian television.
In comments posted in English on Monday, she repeated the assertion, saying that witness testimony made it appear that some chemical weapons had been used.
"What appears to our investigation is that it was used by the opponents, by the rebels," she said. "We have no indication at all that the Syrian government has used chemical weapons."
"THE AIM IS TO TERRORISE"
The Geneva-based inquiry into war crimes and other human rights violations led by Brazilian expert Paulo Pinheiro is separate from an investigation of the alleged use of chemical weapons instigated by UN Secretary-General Ban Ki-moon.
Medecins Sans Frontieres (Doctors Without Borders), a medical charity that runs four hospitals in rebel-held areas of Syria, including Aleppo, has not come across patients suffering from the use of chemical weapons, spokeswoman Emma Amado said.
In Washington, a U.S. official said: "Our understanding has been that the armed opposition does not have such weapons and so we'll have to re-check our facts but our initial take on that was that they do not have such things in their arsenal.
Turkey has also been testing blood samples taken from Syrian casualties brought to a Turkish hospital to determine whether they were victims of a chemical weapons attack, but the results have not yet been made public.
Wassim Taha, a Syrian doctor from the Union of Syrian Medical Relief Organisations which runs hospitals for the Syrian opposition, said initial tests on the patients at a rudimentary laboratory at the Syrian border post of Bab al-Hawa revealed signs of what could have been the use of "diluted sarin".
"The patients' symptoms, which included choking, foaming at the mouth, contraction of pupils are all symptoms that indicate a chemical gas was used," he told Reuters by telephone from Reyhanli, on the Turkish side of the border with Bab al-Hawa.
"The symptoms are less severe (than undiluted sarin), it does not kill a large number of people and does not stay in the atmosphere for a long period of time. The aim is to terrorise and scare people," he said.
