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".