Amid on-going uncertainty over who used chemical weapons in Syria and when, there have been claims and counterclaims that analysts say are difficult to verify.
Western experts believe Syria has one of the world’s largest chemical weapons arsenals, including mustard gas, the more modern sarin and even VX - the most toxic of all chemical agents.
But Greg Thielman, an expert with the private Arms Control Association, said the Syrian government lacks a track record of using the weapons.
“Unlike Iraq, and the history of Iraq, we don’t have a pattern of Syrian usage of chemical weapons over the years," he said.
"This isn’t to suggest that they have a benign regime,” Thielman said. “It’s just that it was Iraq that set new post-war records for massive uses of chemical weapons against Iran in that long war and then against some of its people in the infamous  attack on the village of Halabja, for example.”
But British, French and Israeli officials have recently charged that Syrian President Bashar al-Assad has used small amounts of chemical weapons against rebel forces.
Charles Blair, a chemical weapons expert with the Federation of American Scientists, said the Israelis base their assessment essentially on aerial photographs and camera footage taken in hospitals where people are said to exhibit signs of sarin poisoning.
“For the British and for the French the proof is interviews. It is also blood samples,” Blair said. “It is believed that it is blood samples that were provided to them by the rebels. So it’s automatically suspect, because obviously the rebels have everything to gain in framing the Assad government.”
The United States has taken a more cautious approach, saying U.S. intelligence agencies “assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria - specifically the chemical agent sarin.”
At the same time, the Obama administration said there needs to be a full investigation of chemical weapons use in Syria.
The United Nations has set up a team to investigate such claims. But the Assad government and the U.N. have not agreed on terms of access. The inspectors are currently in Cyprus
Blair said the U.N. option seems at a dead end.
“And the only other possibility then would be if the U.S. had a covert team that was operative in an area that had an alleged sarin attack and they were able to get in there and get samples quickly, control the sample, control the chain of custody and then take it and verify it in U.S. or British labs and do it that way. But it doesn’t seem to me that the Obama administration really has the appetite for that.”
Experts said one has to be extremely careful about the so-called evidence of chemical weapons use in Syria, whether by the Assad government or by the opposition.
U.N. officials this week scrambled to say they had no conclusive findings after former war crimes prosecutor Carla Del Ponte said there are indications rebel forces had used the nerve agent sarin.
Thielman, the Arms Control Association analyst, said one has to be 100 percent sure of the information before even talking about what to do next in Syria - such as military intervention to secure the stockpiles of chemical weapons.
“Iraq hangs over us as a negative example of getting into an enormously significant commitment on the basis of what turned out to be false information,” he said.
Thielman said “we knew a lot more about Saddam Hussein and his chemical weapons status at the time of our invasion of Iraq, than we do in the case of Syria. And we got it wrong even then.”