A Marshall Islands-flagged cargo ship forced into an Iranian port was ordered to shore because of a commercial dispute, the official Fars news agency said.
US defence officials said at least five Iranian ships demanded that the Maersk Tigris head to Iran's Larak Island while it was in the Straight of Hormuz.
The vessel "had a dispute with the Iranian port administration, which obtained a court order to impound (the vessel)", Fars reported, without providing details.
The cargo ship's captain "declined" the demand to alter course and one of the Iranian vessels "fired shots" across the bow of the Maersk Tigris, Pentagon spokesman Colonel Steven Warren said.
The cargo ship then "complied with the Iranian demand and proceeded into Iranian waters in the vicinity of Larak Island," he said.
The US military's Central Command ordered a naval destroyer to the area and military aircraft were monitoring the situation, Warren said.
The destroyer was directed "to proceed at best speed to the nearest location of the Maersk Tigris", he said.
The incident occurred at about 0900 GMT in the Strait of Hormuz in Iranian territorial waters.
It came amid heightened tensions in the region as Saudi Arabia and its Gulf allies wage a campaign of air strikes in Yemen against Iranian-backed Huthi rebels.
The United States is providing intelligence and aerial refuelling for the Saudi-led coalition.
