The US aviation regulator Monday ordered airlines based in the United States to stop flying over Syria, citing a “serious potential threat” to civil planes, including by armed groups with anti-aircraft weapons.

“Based on an updated assessment of the risk associated with such operations and the lack of any requests from operators wishing to fly in this airspace, we believe it prudent to prohibit US operators from flying into, out of and over Syria,” the US Federal Aviation Administration said in a statement.

The FAA’s previous so-called Notice to Airmen had strongly advised US operators against flying over Syria.

“The ongoing armed conflict and volatile security environment in Syria poses a serious potential threat to civil aviation,” the new notice said.

“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.”

It noted that opposition groups have already shot down Syrian military aircraft over the conflict that began nearly three and a half years ago.

The ban affects all US companies and commercial operators.

The FAA has also imposed a ban on US planes over Iraq, effective August 8.

Syria, like Iraq, is on a path that carriers can take when traveling between Europe and the Middle East or Asia.