The United Kingdom Maritime Trade Operations (UKMTO), which provides security alerts for the shipping trade, said the unidentified vessel was fired on in the Gulf of Aden at 0300 GMT.

The agency's website said three boats carrying three to four people each carried out the "attack" before later downgrading the event, 18 nautical miles south of Nishtun, to an "incident".

Authorities confirmed it as "government agency activity", UKMTO added, without elaborating.

Ambrey, a Britain-based maritime security company, said the vessel was a Cook Islands-flagged yacht that had departed the port of Salalah, in Oman, before entering Yemeni waters.

"She continued her voyage following the event," Ambrey said in a statement to AFP.

The incident comes a day after Iran seized a US-bound oil tanker in the Gulf of Oman, another waterway bordering the Arabian Peninsula. Tehran said the tanker had been involved in a collision with another ship that left two Iranian crew missing.

The Bahrain-based US Fifth Fleet demanded Iran immediately release the tanker, saying its "actions are contrary to international law and disruptive to regional security and stability".

The UKMTO is involved in anti-piracy operations and is part of Britain's Royal Navy, which coordinates surveillance in the region.

Previous attacks have been blamed on Somali pirates but international security operations have reduced these incidents in recent years.

Pirate attacks fell to their lowest level in nearly three decades in 2021, according to the International Maritime Bureau watchdog.

The Gulf of Aden, previously known as Africa's piracy hotspot, has been eclipsed in recent years by the Gulf of Guinea off the continent's west coast.

However, incidents continue off Yemen, the Arabian Peninsula's poorest country, which has been engulfed by conflict since 2015, leaving hundreds of thousands dead and millions displaced.

In January last year, an Emirati-flagged vessel was seized in the Red Sea by Yemen's Iran-backed Huthi rebels, who released what they said were pictures of military equipment on board.

And in October 2021, an Iranian warship repulsed an attack by pirates against two oil tankers that it was escorting in the Gulf of Aden.