A UN team of investigators into rights abuses in Syria has stressed there is no conclusive proof of either side in the conflict using chemical weapons, despite a team member's claims to the contrary.

"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," the commission said in a statement on Monday.

Earlier, Carla del Ponte, a former war crimes prosecutor and a member of the commission, had told Swiss public broadcaster RSI that "according to the testimonies we have gathered, the rebels have used chemical weapons, making use of sarin gas".

She acknowledged there was "still not irrefutable proof, [but] very strong suspicions, concrete suspicions that sarin gas has been used... by opponents, by rebels, not by government authorities."

The Geneva-based inquiry into war crimes and other human rights violations is separate from an investigation of the alleged use of chemical weapons instigated by UN Secretary-General Ban Ki-moon.

Ban's office is still trying to negotiate entry into Syria to investigate and collect samples.

The Free Syrian Army (FSA) swiftly denied the claim that rebel forces had used chemical arms.

Speaking to Al Jazeera, Salim Idris, FSA chief of staff, said he considered the remarks a "huge injustice" and "provocation" to the Syrian people.

The Syrian regime and rebels have both accused each other of using chemical weapons, which are banned under international law.

The opposition Syrian National Coalition (SNC) condemned any use of chemical weapons and said only the regime had such capabilities.

"The Coalition condemns all use of chemical weapons, whoever uses them and it will pursue its investigation and collect evidence on this issue to present to the international commission of inquiry," the SNC said in a statement on Monday.

"If the inquiry proves that any party other than the Syrian regime has used chemical weapons, the coalition will take all legal and appropriate measures whoever the party is and whatever the reason or motive for the usage might be."

US 'sceptical'

Del Ponte's comments come amid growing Western suspicions that Bashar al-Assad's government has used chemical weapons.

The US has said intelligence indicates Syria has used the nerve agent sarin on at least two occasions, but President Barack Obama has stressed that he needs more definitive proof before making a decision about how to respond - and whether to take military action.

The White House said on Monday it was "highly sceptical" of an assertion that Syrian rebels had used chemical

weapons.

"We are highly sceptical of suggestions that the opposition could have or did use chemical weapons," Jay Carney, White House spokesman, said.

"We find it highly likely that any chemical weapon use that has taken place in Syria was done by the Assad regime. And that remains our position."

Del Ponte, 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.

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 the UN says 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.

Originally developed as a pesticide, sarin was used to deadly effect in air raids in 1988 by Saddam Hussein's forces on the Kurdish village of Halabja in northern Iraq that left an estimated 5,000 people dead.