"A suspicious boat with 12 armed people on board approached the Iranian merchant ship in Bab al-Mandab" strait on Thursday, the state news agency IRNA said, citing a statement by the navy.

It said a squadron had come into confrontation with the "pirates in the Red Sea", adding that the invading boat "left the area" after the escort flotilla, "headed by the Jamaran destroyer... opened fire" at the vessel.

The incident comes after the Pentagon said on Tuesday that an Iranian ship seized an American military unmanned research vessel in the Gulf but released it after a US Navy patrol boat and helicopter were deployed to the location.

On August 10, a senior Iranian navy commander said the same naval flotilla thwarted an overnight attack on another vessel belonging to the Islamic republic.

Rear Admiral Mustafa Tajeddini said at the time that, following a help request by an Iranian ship in the Red Sea, the flotilla was dispatched to the scene and engaged fire with the attacking boats.

"After heavy exchanges, the attacking boats made off," he added.

Like other countries dependent on the shipping lane through the Red Sea and Suez Canal, Iran stepped up its naval presence in the Gulf of Aden after a wave of attacks by Somalia-based pirates between 2000 and 2011.