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Iranian naval forces repeatedly aimed a laser at an American military helicopter during a routine flight in international airspace over the Gulf, the US military said Thursday.

The helicopter -- an AH-1Z Viper -- is attached to a unit deployed on the USS Bataan amphibious assault ship, which was sent to the region as part of American efforts to deter seizures of commercial tanker ships by Tehran.

Iranian "vessels shone a laser multiple times at the aircraft while in flight" on Wednesday, spokesman Commander Rick Chernitzer said in a statement.

"These are not the actions of a professional maritime force. This unsafe, unprofessional, and irresponsible behavior by the Iranian Revolutionary Guard Corps Navy risks US and partner nation lives and needs to cease immediately," Chernitzer added.

The US military says Iran has either seized or attempted to take control of nearly 20 internationally flagged ships in the region over the past two years.

There have been a series of such incidents since then-US president Donald Trump pulled out of the Iran nuclear deal and reimposed crippling sanctions on the Islamic republic.