One of the most horrific chemical attacks on a civilian area occurred 25 years ago, when Saddam Hussein unleashed chemical weapons in Iraq's Kurdish city of Halabja. The attack left thousands dead and thousands more wounded.

--What's the risk of Syria using chemical weapons on foreigners?

While Syria has vowed it would never use "unconventional weapons" or weapons of mass destruction against its citizens, it gave a stern warning to other countries who might try to intervene militarily in the Syrian conflict:

"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," Foreign Ministry spokesman Jihad Makdissi said in July.

But with no end in sight to Syria's 21-month conflict, some aren't so sure Syria will keep its promise.

"For the first time in the history of the Chemical Weapons Convention (CWC), which entered into force in April 1997, there are serious concerns that chemical weapons might be used," said Ahmet Uzumcu, director-general of the Organisation for the Prohibition of Chemical Weapons.

U.S. President Barack Obama warned Syria that any deployment of chemical agents would be catastrophic.

"The use of chemical weapons is and would be totally unacceptable," Obama said this week. "And if you make the tragic mistake of using these weapons, there will be consequences, and you will be held accountable."