DUBAI, United Arab Emirates — A Japanese shipping line raised alarm Wednesday when it said that one of its supertankers was damaged by an explosion in the Persian 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 U.S. Navy fleet that patrols the region acknowledged reports of an explosion aboard the ship but said the cause of the blast is unclear.
Port officials said the damage was caused by an unusually strong wave that slammed into the side of the ship.
"We do not think the cause of explosion was due to freak waves," Mitsui spokeswoman Eiko Mizuno said in Tokyo. "Waves can capsize vessels, but not cause explosions."
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.
Al-Qaida has in the past carried out attacks on oil infrastructure on land in nearby Saudi Arabia, as well as a 2002 suicide bombing against a French oil tanker off the coast of Yemen and the 2000 bombing of the USS Cole in the Yemeni port of Aden.
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.
No oil leaked from the supertanker, although some members of the 31-strong crew were injured, said a general manager at the UAE port of Fujairah.
"The cause of the incident was a freak wave and there is damage in the upper accommodation decks of the ship," he said.
The Marshall Islands-flagged tanker, loaded with 270,000 tons of oil, was heading from the petroleum port of Das Island in the United Arab Emirates to the Japanese port of Chiba outside Tokyo, the ministry said. After the reported blast, the tanker made its way the Emirati port of Fujairah under its own power, where it dropped anchor late Wednesday.
The Strait of Hormuz is a vital shipping lane for crude oil, natural gas and other goods headed out of the Persian Gulf. It is far from areas where Somali pirates typically prey on slow-moving ships, though smugglers are known to operate in the area between Iran and an enclave of Oman on the other side of the strait.
The Japanese ministry said none of the country's ships has been attacked by pirates in the area.
Iran has in the past threatened to close the strait if the United States attacks it over Tehran's disputed nuclear program, though there were no immediate signs of Iranian involvement.
The Emirates' official state news agency WAM quoted Fujairah port director Musa Murad as saying the tanker sustained damage when it was hit by a large wave caused by a tremor. WAM separately quoted an "official source" who ruled out the possibility the tanker had been attacked.
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, according to Iran's semiofficial Mehr news agency.
The U.S. Geological Survey said it has not had any reports of recent earthquakes in the area, which is prone to seismic activity. Mizuno said the shipping company had no reason to believe a large wave or earthquake was to blame.
Omani officials couldn't immediately be reached.
The U.S. Navy's Bahrain-based 5th Fleet, which patrols the region, said it is investigating the explosion but does not know what caused it. Initial reports from the ship's owner say one lifeboat was blown off the ship, and some starboard hatches were damaged, according to the Navy. It said it offered to assist the tanker after the explosion but was told no help was needed.
The Associated Press and Reuters contributed to this report.