United Nations investigators say they have found testimony from victims and medical staff that shows militants have used the nerve agent sarin in Syria, which has been classified as a weapon of mass destruction in UN Resolution 687.

The UN Independent Commission of Inquiry on Syria could not find any evidence that Syrian government forces used chemical weapons against militants, commission member Carla Del Ponte said on Sunday, Reuters reported.

    "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 said in television interview.

"This was use on the part of the opposition, the rebels, not by the government authorities," said Del Ponte, a former Swiss attorney-general who also served as prosecutor of the International Criminal Tribunal for the former Yugoslavia.

The Syrian government and the foreign-sponsored militants accuse each other of using chemical weapons three times -- in March once near Aleppo and second time near Damascus, and another time in Homs in December last year.

On December 17, Syrian Ambassador to the UN Bashar Ja’afari said in letters to the UN Security Council and the UN secretary general that the foreign-sponsored militants could use chemical weapons against Syrians and try to shift the blame to the government.

Damascus is "genuinely worried" that Syria’s enemies could provide chemical weapons to armed groups "and then claim they had been used by the Syrian government," Ja’afari stated.

The Syria crisis began in March 2011, and many people, including large numbers of soldiers and security personnel, have been killed in the violence.

The Syrian government says that the chaos is being orchestrated from outside the country, and there are reports that a very large number of the militants are foreign nationals.

In an interview recently broadcast on Turkish television, Syrian President Bashar al-Assad said that if the militants take power in Syria, they could destabilize the entire Middle East region for decades.