Four Iranian Revolutionary Guard vessels sped close to a U.S. guided missile destroyer in international waters Tuesday in an "unsafe and unprofessional" encounter, the U.S. Navy said Wednesday.
The USS Nitze fired 10 warning flares and blew its whistles at the Iranian vessels but got no response.
The four boats came within 275 meters of the Nitze, forcing it to change course, even though oil rigs were dangerously nearby, to avoid a possible collision.
Tuesday's incident occurred in the strategically important Strait of Hormuz.
The Navy accused Iran of harassing the warship and breaking international law.
"These sorts of unsafe and unprofessional actions can lead to escalation and miscalculation, which may necessitate additional defensive measures," a Navy spokesman said Wednesday. "Commanding officers have an inherent obligation for self-defense."
There has been no explanation so far from Iran.
Iran briefly held 10 U.S. sailors captive in January after they accidentally strayed into Iranian waters.