Testimony from victims of the conflict in Syria suggests rebels have used the nerve agent, sarin, a leading member of a UN commission of inquiry has said.
Carla Del Ponte told Swiss TV that there were "strong, concrete suspicions but not yet incontrovertible proof".
Ms Del Ponte did not rule out the possibility that government forces might also have used chemical weapons.
Later, the commission stressed that it had "not reached conclusive findings" as to their use by any parties.
"As a result, the commission is not in a position to further comment on the allegations at this time," a statement added.
The BBC's Imogen Foulkes in Geneva says the statement was terse and shows that the UN was taken by surprise at Ms Del Ponte's remarks.
The Independent International Commission of Inquiry on Syria was established in August 2011 to examine alleged violations of human rights in the Syrian uprising. It is due to issue its latest report next month.
In an interview with Swiss-Italian TV on Sunday, Ms Del Ponte, who serves as a commissioner on the panel, said: "Our investigators have been in neighbouring countries interviewing victims, doctors and field hospitals.
"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."
Sarin, a colourless, odourless liquid or gas which can cause respiratory arrest and death, is classed as a weapon of mass destruction and is banned under international law.
Ms Del Ponte, a former Swiss attorney-general and prosecutor with the International Criminal Tribunal for the former Yugoslavia (ICTY), did not rule out the possibility that troops loyal to President Bashar al-Assad might also have used chemical weapons, but said further investigation was needed.
"I was a little bit stupefied by the first indications we got... they were about the use of nerve gas by the opposition," she said.
Ms Del Ponte gave no details of when or where sarin may have been used.
However, a member of the main Syrian opposition alliance, the National Coalition, denied rebel fighters had done so.
"The claim is unsupported," Molham al-Droubi told the Reuters news agency. "There is no objective evidence."
US officials also said Washington had no information to suggest Syrian rebels had the capability or intention to use Sarin.
Last week, the US and UK have said their own investigations suggest government forces have used chemical weapons. British Defence Secretary Philip Hammond said the evidence was quite compelling, but that it would need to be incontrovertible before the case for an international response could be made at the UN.
On Monday, Russian foreign ministry spokesman Alexander Lukashevich said it was deeply concerned by "signs that world public opinion is being prepared for possible military intervention" in Syria.
On the question of whether chemical weapons had been used, he called for an "end to the politicisation of this issue" and to the "whipping up of an anti-Syrian atmosphere".
Ms Del Ponte's comments might therefore complicate matters for the US Secretary of State John Kerry ahead of his visit to Moscow this week, says BBC diplomatic correspondent Bridget Kendall.
If Mr Kerry was hoping to cite fears that the Assad regime was now using chemical warfare as a reason why the Russians should shift their position, that argument will not be so easy to make, our correspondent adds.
A separate United Nations team was established to look specifically into the issue of chemical weapons.
It is ready to go to Syria but wants unconditional access with the right to inquire into all credible allegations.
Both the Syrian government and the rebels have in the past accused each other using chemical weapons.
The US and the UK have said there is emerging evidence of Syrian government forces having used sarin, with Washington saying it had "varying degrees of confidence" that chemical weapons had been deployed.
President Barack Obama called in April for a "vigorous investigation", saying the use of such weapons would be a "game changer" if verified.
President Assad's government says the claims do not have any credibility, denouncing them as "lies".
Ms Del Ponte's allegations concerning the use of sarin by rebels came after Israel carried out a series of air strikes on Syrian military targets early on Sunday.
The Israeli government made no official comment, but security sources said the strikes were aimed at preventing the transfer of advanced Iranian-made missiles to Lebanon's Shia Islamist movement, Hezbollah.
The Syrian government said the Jamraya military research centre, north-west of Damascus, was hit.
A later statement gave more details, saying military positions in the Jamraya area were struck along with other facilities at Maysaloun, near the Lebanese border, and a military airport at Dimass.
The statement said there was massive damage at those locations and nearby civilian areas with many people killed or injured. It also denied that the targets had included missiles for Hezbollah.
Syrian Deputy Foreign Minister Faisal Miqdad said the Israeli air strikes amounted to a "declaration of war" and threatened retaliation.
The New York Times quotes an unnamed senior Syrian official as saying dozens of elite troops stationed near the presidential palace had been killed. The AFP news agency quoted the Syrian Observatory for Human Rights, a UK-based activist group, as saying 42 soldiers had died and another 100 were unaccounted for.
Images on state TV showed large areas of rubble with many buildings destroyed or badly damaged.
The Arab League condemned the raids and UN Secretary General, Ban Ki-Moon expressed concern.
He said all sides should "exercise maximum calm and restraint" and "act with a sense of responsibility to prevent an escalation of what is already a devastating and highly dangerous conflict".
Russia's foreign ministry warned that the "further whipping-up of armed confrontation" sharply increased the risk of "pockets of tension" in Syria and Lebanon, and along their shared border.