Iranian naval ships fired across the bow of a Marshall Islands-flagged cargo vessel in the Strait of Hormuz on Tuesday, and forced it to head to Iranian territory, US defense officials said.
At least five Iranian ships demanded that the Maersk Tigris, which had no Americans on board, head to Larak Island.
The cargo ship's captain "declined" the demand 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," Warren said.
The US military's Central Command ordered a naval destroyer to the area and military aircraft were monitoring the situation, he 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.
The confrontation came amid heightened tensions in the region as Saudi Arabia and its Gulf allies carry out air strikes in Yemen against Iranian-backed Houthi rebels.
The United States is providing intelligence and aerial refueling for the Saudi-led coalition.