FUJAIARAH, United Arab Emirates (Reuters) - A Japanese supertanker damaged near the strategic Strait of Hormuz was involved in a collision, but the cause was unknown, a Fujairah ports official said on Thursday.

"What we know is some collision happened. We don't know what it was," Captain Mousa Mourad told a news conference.

Japanese shipper Mitsui O.S.K. Lines Ltd said it had hired a specialist on military attacks to help investigate unexplained damage to its vessel the M. Star.

It was damaged early Wednesday while traveling from Qatar with a cargo of crude for Japan.

A photograph of the damage released on Thursday showed a big dent in the side of the tanker.

(Reporting by Amena Bakr; Editing by Jason Neely)