Iranian officials on Friday slammed the interception of an Iranian passenger plane by a US fighter jet in the skies over Syria as “illegal,” threatening action against Washington over the incident.

Iran had said that one of its airliners, flying from Tehran to Beirut on Thursday, was “harassed” by fighter jets, but later landed safely in Lebanon. A U.S. official confirmed a U.S. jet had passed by the Iranian airliner, but at a safe distance.

According to Iranian state TV, two fighter jets came within a distance of 100 meters (328 feet) of the Iranian Airbus A310. The pilot of Mahan Air Flight 1152 quickly took the aircraft to a lower altitude to avoid a collision, the report said. The sharp maneuver caused slight injuries among some of the passengers.

However, U.S. Navy Capt. Bill Urban, a Central Command spokesman, told The Associated Press that a U.S. F-15 fighter jet “conducted a standard visual inspection” of the Iranian plane “at a safe distance of approximately 1,000 meters (3,280 feet) from the airliner.”

He said the inspection was meant to ensure the safety of U.S. coalition troops in al-Tanf in Syria as the plane was flying over that area. He said once the aircraft was identified as a passenger plane, “the F-15 safely opened distance from the aircraft.”

Aircraft at that altitude are to maintain a distance of at least 600 meters (2,000 feet) to ensure they don’t hit each other, though planes traveling that close can encounter wake turbulence.

Iran’s foreign minister, Mohammad Javad Zarif, said Friday that what happened was an act of “lawlessness upon lawlessness.”

Zarif tweeted: “U.S. illegally occupies territory of another State and then harasses a scheduled civil airliner—endangering innocent civilian passengers—ostensibly to protect its occupation forces.”

Iran’s Transportation Minister Mohammad Eslami described the incident as a “terrorist act” and said Tehran would complain to the International Civil Aviation Organization. “We expect this to lead to condemnation of the terrorist act by the U.S. government,” Eslami said.

The ICAO said Friday it had not yet received a complaint from Iran.

Lebanon’s militant Hezbollah group echoed Elsmai and said the close encounter could have had “incalculable ramifications” across the region. The Iran-backed group, which has militiamen fighting in Syria on the side of President Bashar Assad’s forces, called the U.S. an occupier of Syrian skies and territories.

The interception comes amid heightened tensions between Iran and the U.S. after President Donald Trump unilaterally withdrew from Tehran’s nuclear deal with world powers over two years ago.

In the time since, there have been several incidents across the Mideast between the two countries, including the U.S. airstrike that killed an Iranian general in Baghdad and Tehran launching ballistic missiles that targeted American forces in Iraq.