In order to be sure of what happened, it's key to get people on the ground quickly who can speak to any alleged attack victims, study medical records and analyze samples before they decompose, said Lewis.

The United Nations investigation team will be sent to Syria "as soon as practically possible," U.N. Secretary-General Ban Ki-moon said, but it's a complex mission that will not happen overnight.

U.S. officials remain to be convinced that the claims of chemical weapons use either in Khan al-Asal or in the rural Damascus suburb of Ateibeh are credible.

U.S. President Barack Obama and other American officials have said they are "deeply skeptical" of Syrian government claims that the opposition used chemical weapons.

Analysts are also "leaning hard away" from the notion that Syria used chemical weapons against its own people, a U.S. military official directly familiar with the preliminary analysis told CNN.

Pictures of the supposed victims of the chemical weapons attacks do not show the signs of burns or other symptoms that one might expect to see as a result of sarin, mustard gas or VX use, Esfandiary said.

What's to stop Syria from using its reputed arsenal?

The use of chemical weapons could be the final straw that pushes the international community to intervene directly in the two-year-long conflict.

After the latest claims, Obama reiterated his warning to Syria's government that it would be held accountable for the use of chemical weapons "or their transfer to terrorists."

He first warned last August that any sign of chemical weapons being moved around in large quantities or utilized would be "a red line for us."

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."