Armed groups in Syria have an estimated several hundred portable anti-aircraft missiles that could easily be diverted to extremists and used to destroy low-flying commercial planes, according to a new report by a respected international research group.
It cites the risk that the missiles could be smuggled out of Syria by terrorists.
The report was released just hours after the Federal Aviation Administration issued a notice yesterday to US airlines banning all flights in Syrian airspace.
The agency said armed extremists in Syria are "known to be equipped with a variety of anti-aircraft weapons which have the capability to threaten civilian aircraft."
The agency had previously warned against flights over Syria, but had not prohibited them.
Small Arms Survey, a Switzerland-based research organisation that analyses the global flow of weapons, published its findings following last month's lethal missile attack on a passenger jet flying over Ukraine.
The report focuses on launchers and missiles known as "man-portable air defense systems," or MANPADS, which are dangerous to planes flying at lower altitudes or ones taking off or landing.
The new report estimated that several hundred anti-aircraft missile systems are already in rebel arsenals. Mostly Russian and Chinese in origin, the weapons have been seized by Syrian opposition militias from government forces and smuggled in from nations sympathetic to the insurgents, the report said.
The most immediate danger is that anti-aircraft weapons, especially newer and sophisticated models, could easily be diverted to extremist groups operating outside Syria, it said.
Porous borders and the presence in Iraq and other neighboring countries of groups affiliated with al-Qaida and other extremists heighten the danger that anti-aircraft weapons could spread to other trouble spots.
"In the hands of trained terrorists with global reach, even a few missiles pose a potentially catastrophic threat to commercial aviation," wrote Matthew Schroeder, the report's author.
The analysis is based on government and media reports and video footage of anti-aircraft weapons posted online from inside Syria.
The extremist Islamic State group that has overrun much of northern and western Iraq also operates inside Syria. The militants, who have drawn fire from US drones and fighter jets, recently posted an online propaganda video showing one fighter appearing to fire an older-model, Russian-made SA-7 missile system.