In the key Strait of Hormuz, the US Navy reported its forces had a tense encounter with Iran's paramilitary Revolutionary Guard.
Three Coast Guard vessels had a "unsafe and unprofessional" interaction, according to the Navy's 5th Fleet in the Middle East.
According to the Navy, one of three Guard swift crafts collided with the Sirocco before reversing course.
During the incident, the Sirocco also shot a warning flare, according to the Navy.
The Navy published a short video of the encounter. The event in the strait, across which a fifth of the oil transported travels, was not immediately acknowledged by Iran.
It comes as tensions in the region remain high as Iran's nuclear deal with international powers remains in shambles, with discussions halted.