By Jonathan Saul

LONDON, Aug 18 (Reuters) - Ships should adopt heightened
vigilance when transiting the Strait of Hormuz after a Japanese
tanker was damaged in the waterway and indications point to a
possible attack, the ship's flag registry said on Wednesday.
The 333-metre M.Star tanker, owned by Japan's Mitsui O.S.K.
Lines Ltd. and registered in the Marshall Islands, was damaged
last month by a mystery explosion while travelling near to the
Strait, which handles 40 percent of the world's seaborne oil.

The Marshall Islands, which is the world's third-largest
open ship registry, issued an advisory this week urging ships to
"exercise the highest level of vigilance and caution,
particularly during night transits with increased monitoring of
small vessel and boat activity".

"We cannot positively confirm that it was a terrorist attack
at this time, but the indications would imply that that might be
what took place," Thomas Heinan, Deputy Commissioner of Maritime
Affairs with the Republic of the Marshall Islands, told Reuters
on Wednesday.

"And as a result we needed to advise ship owners and
operators what to watch out for and what to do if a ship finds
itself in a similar situation," he said in an interview.

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.

"Although the incident is still under investigation, it can
be said that the very suspicious activity of two small craft
observed on the vessel's radar and recorded by the ship's voyage
data recorder and subsequent forensics tend toward supporting
the belief that an unsuccessful terrorist bombing attempt was
made on the M. Star," the Marshall Island's advisory said.

INVESTIGATION

Some security analysts were sceptical of the group's claim,
though the United Arab Emirates state news agency said
investigators had found traces of explosives on the tanker.

This is the first advisory issued for the Strait of Hormuz
by the Marshall Islands, which has sent investigators as part of
the team probing the cause of damage to the M.Star.

"Given the current investigation we felt compelled to
provide additional guidance to ship owners and operators should
they find themselves in a situation where there might be similar
activities or incidences taking place," Heinan said.

The U.S. Department of Transportation's Maritime
Administration issued a similar advisory to vessels last week
urging increased caution around Hormuz.

One crew member was injured in the M.Star incident. There
was no oil spill or disruption to shipping.

Japan's Transport Ministry said on Wednesday checks had
found a soot-like substance in a large dent in the tanker's hull
but it was unclear what caused the blackish substance and it was
being analysed further.

"Our investigation is not yet complete and we have not
received any official findings from any other participating
governments," Heinan said.

He said it had not been able "to glean any definitive
timeline" on how long the investigation would last.

There have been growing concerns for maritime security in
the area. Al Qaeda has threatened to attack shipping there in
the past.

Al Qaeda bombed the USS Cole warship in October 2000 when it
was docked in the southern Yemen port of Aden, killing 17 U.S.
sailors. Two years later an al Qaeda attack damaged a French
tanker in the Gulf of Aden.
(Editing by Alison Birrane)