The U.S. Navy says it prevented an Iranian naval vessel from seizing one of its unmanned surface vessels in international waters in the Persian Gulf overnight on August 29-30.
It said the Iranian Islamic Revolutionary Guards Corps (IRGC) ship was trying to "detain" one of the Fifth Fleet's Saildrone Explorer unmanned surface vessel (USV) but disconnected a towing line and left the area about four hours after the USS Thunderbolt responded.
It said there was no further incident.
In a statement, the U.S. Fifth Fleet's commander called the actions of the IRGC support ship, the Shahid Baziar, "flagrant, unwarranted and inconsistent with the behavior of a professional maritime force."
There was initially no confirmation from Iranian officials.
Saildrone-type vessels are generally used for mapping or collecting data, and the U.S. Navy said the USV technology is "available commercially and does not store sensitive or classified information."
Reports of foiled actions are fairly common in the Persian Gulf, a vital shipping route that is routinely patrolled by U.S. and Iranian military vessels.
Dangerous naval encounters involving Western and Iranian militaries or commercial shippers have eased in recent years despite persistent tensions over sanctions-busting and influence in the region.
Tensions are still high between Iran and the United States over a hobbled nuclear deal and what Washington regards as malign activities by Tehran.