American civilian aircraft have been banned from flying over Syria because of the risk from extremist groups.
The US's Federal Aviation Authority (FAA) said in a statement that such groups were "known to be equipped with a variety of anti-aircraft weapons'.
"Opposition elements" had warned against flying over Syria, it said.
Most American and other commercial airlines had already stopped flying over and into Syria during the past three years of the conflict.
In May last year, the FAA warned against flights over Syria, but on Monday decided to prohibit them altogether.
The issue of aircraft flying over conflict zones has been in the spotlight since Malaysia Airlines flight MH17 was shot down over eastern Ukraine on 17 July.
http://www.bbc.com/news/world-middle-east-28850953