By ADAM SCHRECK | AP

Published: Jul 28, 2010 23:31 Updated: Jul 28, 2010 23:31

DUBAI: A Japanese shipping line raised alarm Wednesday that one of its supertankers was damaged by an explosion in a possible attack in the Gulf, but authorities on both sides of the tense waterway denied that any strike occurred.

Details of what happened as the hulking M. Star tanker steered its way through the strategically sensitive Strait of Hormuz remain murky. The US Navy fleet that patrols the region acknowledged reports of an explosion aboard the ship but said the cause of the blast is unclear.

Local officials cited natural causes, such as an unusually strong wave that slammed into the side of the ship.

A photo released by the Emirates state news agency WAM after the tanker arrived in Fujairah port for inspections showed a large, square-shaped dent beginning near the waterline on the rear starboard side of the ship's hull.

The incident happened shortly after midnight as the M. Star entered the strait, heading out of the Gulf, Japanese shipping company Mitsui O.S.K. Lines said.

Mitsui said the explosion seemed to be caused by “an attack from external sources” while the tanker passed through Omani waters in the western part of the vital waterway, a narrow chokepoint between Oman and Iran at the Gulf's mouth.

“We believe it's highly likely an attack,” Mitsui spokeswoman Eiko Mizuno said. “There is nothing that can explode in that part of the vessel.” One of the ship's 31 crew members noticed a flash of light right before the explosion, she said, suggesting something may have struck the vessel. The explosion occurred at the back of the tanker, near an area where lifeboats are stored, causing cuts to a crew member who was struck with broken glass.

If the tanker was attacked, it would be a rare assault on a merchant ship in the Gulf or at the Strait of Hormuz, a transit point for about 40 percent of oil shipped by tankers worldwide.

Yuki Shimoda, an official at Japan's Ministry of Land, Infrastructure, Transport and Tourism, said the ministry did not immediately suspect an attack, but added that the possibility cannot be ruled out.

The tanker, loaded with 270,000 tons of oil, was heading from the port of Das Island in the United Arab Emirates to the Japanese port of Chiba outside Tokyo.

Fujairah port director Musa Murad said the tanker sustained damage when it was hit by a large wave caused by a tremor. Ataollah Sadr, an Iranian shipping official, also said the damage was likely caused as a result of an earthquake and rejected the possibility of a terrorist attack.

The US Geological Survey said it has not had any reports of recent earthquakes in the area. Mizuno said the shipping company had no reason to believe a large wave or earthquake was to blame. ¬