Abu Dhabi, United Arab Emirates (CNN) -- An alleged explosion Wednesday aboard an oil tanker in the Strait of Hormuz near Oman was described Thursday as a "mysterious incident, whose causes are not yet known" by WAM, the United Arab Emirates' state-run news agency.
The Japanese-owned, Marshall Islands-flagged M. Star "suffered hull damage caused by an explosion which seemed to be an attack from external forces," Mitsui O.S.K. Lines Ltd., the tanker's owner and operator, said in a statement.
Mitsui said it directed the ship to the port of Fujairah, United Arab Emirates for investigation and repairs. The boat arrived in Fujairah at 6:40 p.m. Wednesday, the port's marine department said.
WAM reported that authorities have begun "investigating causes shrouding the accident."
Mitsui had sent a letter to the port of Fujairah's harbormaster requesting permission to dock and stating the condition of the vessel. Mitsui said the ship was "moving under her own power without any assistance." In addition, Mitsui told the harbormaster the only breach of the hull was four meters above the water, too high for water to infiltrate the boat, and the vessel was not polluting the waters.
But though the ship was described as "completely stable and seaworthy and proceeding safely," Mitsui noted in its letter that the alleged explosion had caused significant damage.
"Starboard side all doors to the accommodation deck were damaged/blown away. Paneling inside the alleyway has been dislodged. Starboard life boat and one life raft on the starboard side is blown away. Buckling and a crack only on the external hull have been observed...," the letter said.
The M. Star was bearing two million barrels of oil picked up at two other ports on the Arabian Peninsula, according to the port of Fujairah marine department. Mitsui said its tanker with 31 crew members -- 16 from the Philippines and 15 from India -- on board, was heading to the port of Chiba, Japan.
The U.S. Fifth Fleet said the strait remained open despite the event.
Capt. Musa Murad, director of the port of Fujairah, gave a different account of the incident. He told WAM that the tanker was passing through the strait when it was exposed to a high wave as a result of an earthquake shock.
A search of the U.S. Geological Survey's global earthquake database shows the last earthquake to strike the region was a 4.8-magnitude temblor near the coast of southern Iran on Saturday. The Strait of Hormuz is a narrow body of water between Oman and Iran, leading from the Gulf of Oman to the Persian Gulf. An Omani enclave sits on the southern coast of the strait. Iranian media quoted a regional official as saying the tanker was damaged in Oman's territorial waters.
Ali Akbar Saffai, head of the marine department of Hormozgan province in Iran, rejected reports that the event occurred in the strait. "The fire, which was triggered by an explosion on the deck of the vessel, was contained with the help of the crew and regional forces," Saffai said Wednesday.
Because of its location, the explosion was unlikely to have been caused by oil on the tanker, Japan's Ministry of Land, Infrastructure, Transport and Tourism said Wednesday.
The U.S. Navy offered emergency assistance to the vessel, but the M. Star's master determined it was not needed, the Fifth Fleet said Wednesday. It added it would continue to monitor the situation and the ship's transit to Fujairah.
Mitsui said the incident happened around 12:30 a.m. Wednesday.
CNN's Junko Ogura, Caroline Faraj and Vicky Brown contributed to this