Shots were fired at the unidentified vessel in the Gulf of Aden, said United Kingdom Maritime Trade Operations, which provides security alerts for the shipping trade.

"UKMTO have received a report of a vessel under attack," the body said, adding "shots fired at vessel". Three boats were involved in the attack with three to four people on board each, the brief statement said.

No further details were given. The report comes a day after Iran seized a US-bound oil tanker in the Gulf of Oman, another waterway bordering the Arabian Peninsula, saying it was involved in a collision that left two crew missing.

The US Fifth Fleet demanded the immediate release of the tanker, saying "Iran's 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 they have fallen in recent years as a result of international security operations.

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.

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

In January 2022, 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.