A US Navy warship fired a warning flare to stop an Iranian Revolutionary Guard speedboat coming straight at it during a tense encounter in the strategic Strait of Hormuz, officials said Tuesday.
The incident on Monday comes as tensions remain high over stalled Iran nuclear negotiations.
Tehran enriches uranium closer than ever to weapons-grade levels under decreasing international oversight.
Meanwhile, the United Nations' nuclear watchdog said Iran now plans to enrich uranium through a second set of advanced centrifuges at its underground Fordo facility amid the standoff.
The US Navy ships found themselves in a close encounter with three Iranian fast boats while coming through the Strait of Hormuz to enter the Persian Gulf, the Navy said.
The overall encounter lasted about an hour, it said.
The Guard's "actions did not meet international standards of professional or safe maritime behavior, increasing the risk of miscalculation and collision," the Navy said.
Iran did not immediately acknowledge the incident.
The Navy separately told The Associated Press that this marked the second so-called "unsafe and unprofessional" incident it had with Iran in recent months.