US carriers have been banned from overflying Syria under a tightening of FAA restrictions on operations in the region.
The FAA had already advised US airlines against operating inside the Damascus flight information region, which encompasses the country.
But it says in a new NOTAM that, owing to the "potentially hazardous situation" created by armed conflict, all US flight operations in the FIR are "prohibited".
It adds that the decision is based on an "updated assessment" of the risks, but it also points out that there has been a lack of requests from operators to fly within the affected airspace.
Limitations have been imposed on a number of airspace regions, including Iraq and Libya, in the wake of the destruction of Malaysia Airlines' flight MH17 over eastern Ukraine on 17 July.
"The ongoing armed conflict and volatile security environment in Syria poses a serious potential threat to civil aviation," says the FAA. "Armed extremist groups in Syria are known to be equipped with a variety of anti-aircraft weapons which have the capability to threaten civilian aircraft.
"Opposition groups have successfully shot down Syrian military aircraft using these anti-aircraft weapon systems during the course of the conflict. Opposition elements have previously warned civilian air carriers against providing service to Syria."