A U.S. Navy patrol boat fired warning shots Tuesday near an Iranian vessel that came close to it during a tense encounter in the Persian Gulf, an American defense official said.

The incident involving the USS Thunderbolt, a Cyclone-class patrol ship based in Bahrain as part of the U.S. Navy’s 5th Fleet, is the latest confrontation between Iranian vessels and American warships.

The Thunderbolt was taking part in an exercise with American and other coalition vessels when the Iranian patrol boat approached it, the official said. The Iranian ship did not respond to radio calls, flares and sirens as it came within 150 yards (137 meters) of the Thunderbolt, forcing the U.S. sailors aboard to fire the warning shots, the official said.

The Iranian boat went “dead in the water” after the shots and the vessels all left the area without further incident, the official said.

The official spoke on condition of anonymity as the incident had yet to be made public. Iranian officials and state-run media there did not immediately acknowledge the incident.

Iran and the U.S. frequently have tense naval encounters in the Persian Gulf. The U.S. Navy recorded 35 instances of what it describes as “unsafe and/or unprofessional” interactions with Iranians forces in 2016, compared to 23 in 2015.

Of the incidents last year, the worst involved Iranian forces capturing 10 U.S. sailors and holding them overnight. It became a propaganda coup for Iran’s hard-liners, as Iranian state television repeatedly aired footage of the Americans on their knees, their hands on their heads.

Iranian forces view the American presence in the Gulf as a provocation by itself. They in turn have accused the U.S. Navy of unprofessional behavior, especially in the Strait of Hormuz, the mouth of the Persian Gulf through which a third of all oil trade by sea passes.

(AP)