The Russians, who have been staunch allies of Syrian President Bashar al-Assad, have indicated that the use of chemical weapons would be a step too far.
Ban also set out the U.N. position clearly, saying his announcement of a U.N. investigation "should serve as an unequivocal reminder that the use of chemical weapons is a crime against humanity."
The relative success of the Chemical Weapons Convention has put their possession and use well outside the norm, experts say. This stigma makes it easier for the United States and others to pressure al-Assad on this point.
However, the muted international response when his forces have used a variety of conventional weapons against his people may have emboldened him, said Esfandiary.
"The best way the West can react is to continue to make their 'red lines' absolutely clear," she said. "The danger is if you get to a situation where Assad really has got nothing to lose, then he really won't care. To put it simply, he won't use them until the very, very last minute."
Lewis, the Chatham House fellow, agrees that the stakes are too high for al-Assad's forces to use chemical weapons lightly, which adds to her skepticism of the rebels' latest claims that they were used near Damascus.
Could they fall into the wrong hands?
Reports have repeatedly surfaced in recent months of Syrian forces moving some of the chemical weapons inventories, possibly because of deteriorating security in the country.
This has raised fears the stockpiles could fall into the hands of al Qaeda-linked groups working with the opposition, should al-Assad's government fall.
Russian Foreign Minister Sergei Lavrov said in December that Syria had consolidated its chemical weapons into one of two locations in the face of ongoing rebel gains.
Esfandiary says she believes it's unlikely that any unconventional munitions have fallen into rebel hands so far.
"Keep in mind that this is Assad's most prized possession. How likely is it that he's going to hand over such important stockpiles to his enemies?" she said.
One disturbing possibility, though -- raised by the opposition -- is that the Syrian government would accuse rebel forces of using them in order to justify then retaliating in kind.
What does Syria say?
Syria had always denied having chemical weapons. But last July, then-Foreign Minister Jihad Makdissi confirmed for the first time that Damascus has "unconventional" weapons, but vowed they "would never be used against civilians or against the Syrian people during this crisis at any circumstance."
"All the stocks of these weapons that the Syrian Arab Republic possesses are monitored and guarded by the Syrian army. These weapons are meant to be used only and strictly in the event of external aggression against the Syrian Arab Republic," Makdissi said. He has since fled to the United States as a refugee, according to the U.S. envoy to Syria, Robert Ford.
In a letter to the U.N. secretary-general in December, Syria said the United States falsely accused it of using chemical weapons.
"What raises concerns ... is our serious fear that some of the countries backing terrorism and terrorists might provide the armed terrorist groups with chemical weapons and claim that it was the Syrian government that used the weapons," the state-run news agency SANA reported.
It was its formal request to the U.N. secretary-general that prompted the promise of an independent investigation into their alleged use.