"More than 80 percent of oil tankers coming to Japan go through that area. An incident like this in such a region is a grave concern for us," Transport Minister Seiji Maehara told the opening session the same day of a committee set up to investigate the cause of the damage.

It was unclear yet what caused the blackish substance, which was spread in a radial pattern, and it would be analysed further, a ministry official told reporters.

Public broadcaster NHK said on the previous day that the tanker's radar detected a small ship that made suspicious movements near it at the time of the incident, and that the Transport Ministry believed there was a possibility that ship launched an attack.

The incident, shortly after midnight on July 28, injured one seaman but caused no oil spill or disruption to shipping in the strategic waterway, which is the gateway to the oil-producing Gulf and handles 40 percent of the world's seaborne oil.

A militant group called Abdullah Azzam Brigades, which is linked to al Qaeda, said that a suicide bomber belonging to it had attacked the tanker. The United Arab Emirates state news agency said investigators had found traces of explosives on the tanker.

A senior official with the Marshall Islands, where the M.Star is registered, said it had issued an advisory urging ships to adopt heightened vigilance when transiting the Strait of Hormuz.

Reuters