United States president Barack Obama has warned Syria of "enormous consequences" if it were to move or use its chemical weapons.
Mr Obama says he has not ordered military intervention in Syria at this point, but that the US was monitoring the situation carefully and had made contingency plans.
There is growing unease that weapons belonging to President Bashar al-Assad's government may fall into the wrong hands as fighting intensifies, the BBC reports.
Speaking to reporters at a White House briefing, Mr Obama said the deployment or use of biological weapons would widen the conflict in the region.
"It doesn't just include Syria. It would concern allies in the region, including Israel, and it would concern us."
Mr Obama warned Mr Assad and "other players on the ground" about the use or movement of such weapons.
"A red line for us is [if] we see a whole bunch of chemical weapons moving around, or being utilised. That would change my calculus."
Syria holds the world's fourth-largest stockpile of chemical weapons. In July, a Syrian foreign ministry spokesperson said the weapons would never be deployed inside the country.