Recent reports have circulated that Syrian forces may deploy chemical weapons as a civil war raging since early last year descends on Damascus. Recent fighting has prevented travel from the international airport in the capital.
Arab broadcaster al-Arabiya transcribed a video message this week said to be from opposition groups in Syria advising civilians how to protect themselves against a weapons attack.
U.N. Secretary-General Ban-Ki moon wrote to Assad calling on him to refrain from using such weapons, his spokesman said. Any such deployment, he added, would be considered "an outrageous crime" that would be met "with dire consequences."
NATO allies this week agreed to send Patriot missiles to Turkey to help the member state protect its borders from Syrian violence. The alliance said there are no plans for a no-fly zone over Syrian airspace, however.
U.S. and Russian officials met this week to discuss a possible solution to the crisis. Russia has stood by its allies in Damascus but has expressed concern about the violence.
White House spokesman Jay Carney said that the Kremlin was united with Washington on the use of chemical weapons.