DUBAI (Reuters) - Armed men boarded a vessel at anchor in the Gulf of Oman on Tuesday 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 had earlier said the vessel had been at anchor around three nautical miles (nm) off Ras Al Kuh.

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Maritime security firm Dryad Global separately tweeted that a Hong Kong-flagged vessel had been released. It earlier said the ship 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".

(Reporting by Lisa Barrington and Nayera Abdallah; Writing by Aziz El Yaakoubi; Editing by William Maclean and Gareth Jones)