Syrian rebels have used the deadly nerve agent sarin in their fight against President Bashar al-Assad's government, according to evidence from victims and doctors, United Nations human rights investigator Carla del Ponte said.

"Our investigators have been in neighboring 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," 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.

"This was use on the part of the opposition, the rebels, not by the government authorities," she added, speaking in Italian.

But on Monday, the team stressed that there was no conclusive proof of either side in the conflict using chemical weapons.

"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.

Del Ponte's comments come amid growing Western suspicions that the Syrian government has used chemical weapons in the 26-month conflict and follow Israeli raids on military sites near Damascus over the weekend.

"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.

Instead, it has interviewed more than 1,500 refugees as a basis for its reports and its charges that both the government forces and their allies and opposition forces carried 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.

AFP-AP-Reuters