The United States issued a fresh warning on Sunday about the risks of using Iranian airspace, claiming military activity there may put civilian and commercial aircraft in danger.

New guidance from the Federal Aviation Administration (FAA) cited the interception of an unidentified U.S. civil operator by Iranian fighter jets last December. The advisory also warned of military activities linked to the ongoing conflict in Syria taking place across Iranian airspace. Iran is currently allowing Russian war jets to use its airspace to help target Syrian rebels.

The Flight Service Bureau, which provides guidance to commercial airlines on the safety of global airspace, said that airlines must consider rising tensions between the U.S. and Iran when planning flights in and around Iranian airspace.

“Although the reopening of Iraqi airspace in November last year has provided additional routing options … there is no perfect route in the region, and operators must consider their preference for Iraq vs Iran,” the group said in an email to clients on Monday.

The bureau also cited the potential risk of whether an American airliner was forced to land in the country for medical or technical reasons, citing a level 4 “Do Not Travel” warning from the U.S. Department of State for all citizens considering entering Iran.

“Do not travel to Iran due to the risk of arbitrary arrest and detention of U.S. citizens. There is a very high risk of arrest and detention of U.S. citizens in Iran, particularly U.S.-Iranian dual nationals,” the warning reads. “Iranian authorities continue to unjustly detain and imprison U.S. citizens, including students, journalists, business travelers, and academics, on charges including espionage and posing a threat to national security.”

Tensions between Iran and the United States have flared in recent months after President Donald Trump reimposed economic sanctions after canceling the Iran nuclear agreement signed under Barack Obama.

The sanctions have already had a major impact on the Iranian economy, with their riyal currency losing 80 percent of its value against the U.S. dollar since January as unemployment and inflation continues to rise. Further sanctions on the country to be put in place in November are only expected to squeeze the regime’s economy even further.

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