"Given the regime's stockpiles of chemical weapons, we will continue to make it clear to Assad and those around him that the world is watching and that they will be held accountable by the international community and the United States should they make the tragic mistake of using those weapons," U.S. President Barack Obama said on Monday.
A U.S. State Department spokeswoman said warnings against using chemical weapons extended not only to the Syrian government but to rebels and any militants who might try to obtain them.
Britain, Germany and other countries also said it was unacceptable for Syria to say it might use chemical arms. U.N. Secretary-General Ban Ki-moon said he was very concerned Syria may be tempted to use unconventional weapons.
Western countries and Israel have expressed fears chemical weapons could fall into the hands of militant groups as Assad's authority erodes. Israel has publicly discussed military action to prevent Syrian chemical weapons or missiles from reaching Assad's Lebanese Shi'ite militant allies Hezbollah.
The Global Security website, which collects published intelligence reports and other data, says there are four suspected chemical weapons sites in Syria: north of Damascus, near Homs, in Hama and near the Mediterranean port of Latakia. Weapons it produces include the nerve agents VX, sarin and tabun, it said, without citing its sources.