Multiple fast-attack vessels from the Iranian Revolutionary Guard Corps came close to a US Navy ship in the Strait of Hormuz on Saturday, forcing it to change direction, a US official told Reuters on Monday.
The official, speaking on the condition of anonymity, said the IRGCN boats came within 600 yards of the USNS Invincible, a tracking ship, and stopped.
The Invincible was being accompanied by three ships from British Royal Navy and forced the formation to change course.
The official said attempts were made to communicate over radio, but there was no response and the interaction was "unsafe and unprofessional."
Lawrence Brennan, a former US Navy captain and an expert on maritime law, said the Invincible is a scientific-research vessel and was unlikely armed except for "small arms for self defense."
The US Navy officially lists the Invincible as a "missile range instrumentation ship" that monitors missile launches and collects data, so it was likely in the region because of Iran's repeated ballistic-missile launches.
The Invincible carries out a mission similar to that of the Russian spy ship that sat outside a US submarine base in Connecticut.
"This generic type of unarmed ship has been a target a number of times," Brennan said, citing attacks on the USS Liberty and Pueblo as examples of similar harassment.
However, the Iranian navy and IRGC navy have made a habit of harassing US ships near the Strait of Hormuz, and experts contacted by Business Insider believe that Iran provided Yemeni Houthi rebels the means to carry out a suicide boat attack on a Saudi Arabian navy vessel that killed two sailors.
This attack made the US Navy acutely aware of the danger from Iran's swarming ships, which the US Navy has resorted to firing warning shots at before.
The US recently tested the efficacy of using A-10 Warthog guns and percision-guided munitions dropped from jets on Iranian fast-attack craft like the ones that harassed the Invincible on Saturday.
More From Business Insider