A senior commander says Iran's navy repelled an attack on an Iranian vessel in the Red Sea overnight on August 9-10, but he didn't identify the type of ship or its attackers.
Rear Admiral Mustafa Tajeddini said a destroyer-led flotilla responded to a distress call "and engaged with the attacking boats."
He said the attackers "made off." The resulting damage was unclear.
Piracy and other minor incidents are not unusual in the oil-rich region, but while seizures and other more serious confrontations are less frequent they can threaten shipping on one of the world's busiest commercial cargo routes.
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 in the region.
Iran boosted its naval presence in the nearby Gulf of Aden after a rash of attacks by Somalia-based pirates that eased about a decade ago.
But Iranian media reported two attacks on Iranian oil tankers in the nearby Gulf of Aden late last year.