DUBAI/OSLO, May 26 (Reuters) - United Kingdom Maritime Trade Operations said on Tuesday that a tanker had reported an external explosion on the vessel's port side, close to the waterline, 60 nautical miles off Oman's capital Muscat.

UKMTO said the vessel, identified as the Olympic Life, and its crew were safe, although the tanker reported some bunker fuel had been discharged into the sea. It said the cause of the incident was unknown.

According to MarineTraffic vessel-tracking data, the Greek-owned Very Large Crude Carrier was sailing past Muscat at around 0700 GMT, heading out of the Gulf of Oman, and was not carrying cargo.

The ship's technical manager, Springfield Shipping, said the vessel was struck by an unidentified object at about 0920 GMT, but remained stable and operational.

"An initial assessment indicated that there was damage to one of the vessel's bunker tanks. A sheen was reported in the water after the incident," it said in a statement to Reuters, adding that the spill had since been contained.

The vessel is owned by Athens-based Olympic Shipping & Management, the successor to Olympic Management founded by late shipping magnate Aristotle Onassis, according to the Onassis Foundation's website.

The company was not immediately available for comment.

(Reporting by Ahmed Elimam and Jana Choukeir; Additional reporting by Nerijus Adomaitis in Oslo; Editing by Bernadette Baum, Alison Williams, Rod Nickel)