Five Iranian boats fired shots across the bow of a Singapore-flagged cargo vessel on Thursday, CNN reported, citing an US official.
It was unclear if the shots hit the vessel, CNN said.
This is not the first time Iranian authorities have showed hostility to trading ships in the Persian Gulf.
Iran seized a Marshall-Islands flagged container ship and its crew on April 28 in the Strait of Hormuz, one of the world's major oil shipping lanes.
The ship was seized by Iranian patrol boats, prompting the United States to send vessels to monitor the situation and to accompany US-flagged vessels passing through the strait.
Iran released the ship later on April 28.
The Strait of Hormuz is a strategic location, through which around 40 percent of the Middle East’s petroleum passes.
Iran has repeatedly threatened to close the strait in the event the country comes under attack.