Armed men boarded a vessel at anchor in the Gulf of Oman but the ship was later released, the United Kingdom Maritime Trade Operations (UKMTO) said.
“Vessel and crew are safe,” it said in an updated advisory, urging all vessels in the vicinity to remain vigilant.
UKMTO earlier said the vessel was at anchor three nautical miles off Ras Al Kuh.
Maritime security firm Dryad Global separately tweeted a Hong Kong-flagged vessel was released. It earlier said the ship was at anchor waiting to approach Jubail in Saudi Arabia.
“At 12:48 UTC the vessel was shown underway towards the Iranian coastline. At 14:28UTC the vessel was stationary at a position 4nm off Mogh-e Qanbareh-ye Kuh Mobarak,” Dryad’s report said.
It said the vessel was “likely to be the vessel of concern”, but added it was “unclear whether the vessel is in distress or being assisted by Iran in some way”.