An Iranian naval flotilla briefly seized two American military unmanned research vessels in the Red Sea before releasing them, Iranian state media reported on Friday.

The Iranian navy's "Jamaran destroyer encountered several American military unmanned research vessels on the international shipping route on Thursday while carrying out a counter-terrorism mission in the Red Sea," Iranian state TV said.

It added that the flotilla, "after warning an American destroyer twice, seized the two drone vessels to prevent possible accidents".

"After securing the passage of international shipping, the flotilla released the two vessels in a safe area," the state broadcaster continued, airing footage purporting to show the two US vessels being released by Iranian forces on board a ship.

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

The US Central Command's 5th Fleet said a support ship from Iran's Islamic Revolutionary Guard Corps Navy, the Shahid Baziar, was spotted towing the seven-metre (23-foot) Saildrone Explorer unmanned surface vessel (USV) late Monday.

US forces then sent the USS Thunderbolt coastal patrol ship and an MH-60S Sea Hawk helicopter.

That "resulted in the IRGCN vessel disconnecting the towing line to the USV and departing the area approximately four hours later" without further incident, the 5th Fleet said.

Separately on Thursday, Iran's Navy said the same flotilla had foiled a pirate attack on an Iranian merchant vessel in the area.

"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.

On August 10, a senior Iranian navy commander said the same flotilla had 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.