A UN investigator says testimony from victims and doctors suggests Syrian rebels have used the deadly nerve agent sarin in their fight against president Bashar al-Assad's regime.

UN human rights investigator Carla del Ponte, a former war crimes prosecutor, made the comments during an interview with Swiss radio.

"According to the testimonies we have gathered, the rebels have used chemical weapons, making use of sarin gas," she said.

She said there was "still not irrefutable proof, (but) very strong suspicions, concrete suspicions that sarin gas has been used".

"Assistance to victims shows this," she said.

Her comments follow Israeli air strikes on military sites near Damascus on Sunday and come amid suspicions that the Assad regime has used chemical weapons in the 26-month conflict.

Ms del Ponte said the UN commission of inquiry on Syria, of which she is a member, is far from finishing its probe.

"We still have to deepen our investigation, verify and confirm (the findings) through new witness testimony," she said.

"But according to what we have established so far, it is at the moment opponents of the regime who are using sarin gas.

"This is not surprising since the opponents have been infiltrated by foreign fighters."

Ms del Ponte also said the commission might still find proof that the Syrian regime was also using this type of chemical weapon.

The UN commission of inquiry on Syria released a statement after Ms del Ponte's comments stressing there was no conclusive proof yet 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.

US president Barack Obama has said the use of chemical weapons in the Syrian conflict is a "red line" for his administration.

But he has also 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 UN commission of inquiry into Syria has so far been unable to gain access to the country as Damascus has ignored repeated requests for entry.

Instead, it has interviewed over 1,500 refugees and exiles as a basis for its reports.

It 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 violence exploded in March 2011.

Nervous system crippled

Sarin is a powerful neurotoxin which was developed by Nazi scientists in the 1930s.

Originally developed as a pesticide, sarin was used to deadly effect in the 1988 raid on the Kurdish village of Halabja in northern Iraq.

A Japanese cult also used sarin in two attacks in the 1990s.

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.

AFP