Warnings from an international research group and the Federal Aviation Administration underscore the rising threat to commercial aircraft posed by hundreds of anti-aircraft weapons that are now in the arsenals of armed groups in Syria and could easily be diverted to extremist factions.

Armed groups opposing the Assad regime in Syria have already amassed an estimated several hundred portable anti-aircraft missiles that are highly mobile, difficult to track and accurate enough to destroy low-flying passenger planes, according to a new report by Small Arms Survey, a respected Switzerland-based research organization that analyzes the global flow of weapons.

The report was released Tuesday, hours after the Federal Aviation Administration issued a notice Monday to U.S. 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. […]