Iran is using a commercial airliner with ties to the country’s Revolutionary Guard Corps (IRGC) to ferry fighters and weapons to Syria.

Sanctioned by the United States for its support of terrorism, Mahan Air operates regular flights from Tehran to Dusseldorf and Munich. But now German politicians are seeking to ban the airline for its alleged ties to Iran’s regime.

With a fleet of over 50 aircraft, Mahan Air has been making secret trips to Syria since August 2015 and has been delivering weapons and fighters from Iran, Iraq and Lebanon to support and reinforce Syrian President Bashar Al Assad’s forces, Germany’s Bild newspaper reported.

Unlike the flights Mahan Air offers to Germany, the airline has no official flights listed to Syria.

Mahan Air's Syria Express is in the air:

Flight W5142 on its way to Damascus for #IRGC airlift to #Assad pic.twitter.com/eFUffoGrmg — Emanuele Ottolenghi (@eottolenghi) May 17, 2016

Emanuele Ottolenghi, a senior fellow with the Foundation for Defense of Democracies has documented numerous Mahan Air flights over the past several months using global flight trackers which show the airline making stops in Syrian cities like Damascus and Latakia and also flying to Baghdad from the Iranian cities Tehran and Abadan, an IRGC logistical hub.

Last week, Syrian activists filmed a Mahan Airbus A310, easily identifiable by its distinctive green and white tail, flying over Huraytan, a city north of Aleppo.

The airline’s practices and close ties with the IRGC have led German foreign policy spokesperson and member of parliment, Omid Nouripour, to urge German and European Union (EU) politicians to investigate and take action against Mahan Air.

In 2011, the U.S. Treasury sanctioned Mahan Air for colluding with the Iranian government’s elite forces and Lebanon’s Hezbollah, an Iranian terror proxy.

A year later, the Treasury blacklisted Mahan Air’s fleet stating that Iran used “Mahan Air flights between Tehran and Damascus to send military and crowd control equipment to the Syrian regime.”

More recently, the Treasury advised the E.U. to prevent the airline from flying passengers to Europe, reasoning that it uses the same aircraft to deliver support to Syrian forces.