The US is assessing claims that the Islamic State (IS) terrorist group has anti-aircraft missile systems such as man-portable air defence systems (MANPADs) in its arsenal, the US Department of State spokesperson Jen Psaki has said.
The German foreign intelligence agency, BND, reported over the weekend that IS has MANPADs, which can shoot down low-flying aircraft, primarily helicopters.
The US State Department is also evaluating claims that IS militants have used the system to target Iraqi aircraft.
Psaki said: "We're assessing these claims. There's clearly [a] significant potential threat to aviation operating in Iraqi and Syrian airspace due to ongoing fighting.
"Of particular concern is our advanced conventional weapons like MANPADs, but we don't have confirmation of this at that time."
Earlier, US Office of the Secretary of Defense commander Elissa Smith was quoted by RIA Novosti as saying: "We can confirm ISIL (IS) has limited anti-aircraft artillery capability, mostly used in a ground combat role.
"Regardless, we have a thorough process and take all necessary action to mitigate anti-aircraft artillery risks.
"We have no evidence that definitively determines [that IS] possesses or has used MANPADs, and we can't speculate about weapons systems ISIL has captured."
According to BND, the anti-aircraft weaponry might have been manufactured in Bulgaria or China using Russian designs, and some are more than 40 years old and were captured by IS from the Syrian Army.
In September, IS released a guide featuring instructions to successfully bring down AH-64 Apaches and other US warplanes, using surface-to-air missile systems.
The US is primarily using a mix of fighters, attack and remotely piloted aircraft to bomb IS fighters in Iraq and Syria, but has around six Apache attack helicopters stationed at Baghdad International Airport.
As part of Operation Inherent Resolve, the helicopters were also used to launch air strikes on the militants north-east of Fallujah, Iraq, earlier this month.
The US is joined by France, UK, Australia, Belgium, Denmark, and the Netherlands for the aerial campaign against IS in Iraq, while Saudi Arabia, Jordan, Bahrain and the UAE are supporting air strikes in Syria.