White House spokesman Jay Carney said that there were concerns that the government of Syrian President Bashar Assad may resort to chemical weapons as rebel fighting moves closer to Damascus.
"As the regime has lost all legitimacy to lead Syria, and the opposition grows in strength, our concern about the regime's intentions regarding its chemical weapons stockpiles has increased," he said.
Arab broadcaster al-Arabiya transcribed a video message said to be from opposition groups in Syria advising civilians how to protect themselves against a weapons attack.
"Nerve gases are heavier than air, therefore, it is recommended to ascend to the highest floor of a building," one of the advisories read. "However, in case the chemicals were outside the building, all windows and doors should be shut immediately."
Carney warned Damascus that it was under close observation. Should the regime use, or lose control of, chemical weapons, "they will be held accountable by the United States and the international community," he said.
NATO announced Tuesday, meanwhile, that it would deliver Patriot missiles to the Turkish border to help protect the country from Syrian violence.