A U.S. aircraft carrier passed through a strategic waterway separating Iran from U.S.-backed Arab states on Tuesday for the first time since President Donald Trump took office two months ago.
Nearly 20 smaller Iranian boats looked on as the USS George H.W. Bush approached the Gulf through the Strait of Hormuz.
About one-fifth of the world's oil passes through the Strait and there are occasional incidents in the area involving U.S. and Iranian military vessels.
Years of mutual animosity eased when Washington lifted sanctions on Tehran last year after a deal to curb Iran's nuclear goals. But serious differences remain over Iran's ballistic missiles as well as conflicts in Syria and Iraq.
While still a presidential candidate in September, Trump vowed that any Iranian vessels that harassed the U.S. Navy in the Gulf would be "shot out of the water."
U.S. helicopters hovered in a cloudy sky as the carrier sailed through choppy waters as part of a convoy that included a Danish frigate and a French destroyer.
Officers aboard were on alert and tried to communicate with the Iranian vessels, but it was not clear if they responded.
Lieutenant Ian McConnaughey, a U.S. Navy spokesman, said such boats were sometimes laden with explosives.
"While these probably don't have explosives, their behavior can lead to a miscalculation because we don't understand their intentions," he said. "Our intention is never to start anything but if the situation arises, we are within our rights to defend ourselves."
Iran, which sees the Gulf as its backyard and believes it has a legitimate interest in expanding its influence there, has long argued that the region should organize its own security collectively, without outside powers.
Accordingly, Iran uses its sea power in the Gulf to show it will not be cowed by Washington's naval presence, analysts say.
The last serious naval incident occurred earlier this month when a U.S. Navy ship and several fast-attack vessels from Iran's Islamic Revolutionary Guard Corps got within 600 yards (meters) of each other. Tehran and Washington offered conflicting accounts of the incident and blamed each other.
A Pentagon spokesman said such interactions were of concern because they could lead to a "miscalculation or an accidental provocation."
In January, a U.S. destroyer fired three warning shots at four Iranian fast-attack boats near the Strait after they closed in at high speed and disregarded repeated requests to slow down.
A year earlier, Iran freed 10 U.S. sailors after briefly detaining them in the Gulf.