Armed men boarded a vessel at anchor in the Gulf of Oman on Tuesday, the United Kingdom Maritime Trade Operations (UKMTO) said.
"The vessel is now reported to be at anchor approx. 3nm (nautical miles) off Ras Al Kuh, in the vicinity of 25-48N 057-14E," the emailed advisory notice said.
UKMTO added it was monitoring the situation and advised all vessels in the vicinity to stay vigilant and to report any incidents.
Maritime security firm Dryad Global said a Hong Kong-flagged vessel had been at anchor waiting to approach Jubail in Saudi Arabia.
"At 12:48 UTC the vessel was shown to be underway towards the Iranian coastline. At 14:28UTC the vessel was shown to be stationary at a position 4nm off Mogh-e Qanbareh-ye Kuh Mobarak," Dryad's report said.
It said the vessel was "highly likely to be the vessel of concern", but added that it was "unclear whether the vessel is in distress or is being assisted by Iran in some way".