(Reuters)—A suspected militant strike on a Japanese-owned tanker illustrates how vulnerable merchant ships are to attack on the high seas with navies stretched fighting Somali piracy, analysts and shipping sources said on Friday.

There have been growing concerns for maritime security after the United Arab Emirates state news agency said earlier on Friday that investigators had found traces of explosives on the tanker damaged near the Strait of Hormuz last week.

A militant group called the Abdullah Azzam Brigades, which is linked to al-Qaida, claimed responsibility. Security analysts based in the Gulf, some skeptical of the group's claim, said they believed the state news agency report.

While Mitsui O.S.K., the Japanese owner of the 333-meter-long M. Star, could not confirm details of the WAM report pending completion of its own investigation, the latest development has raised risks for ships.

"If it is proved to have been a terrorist case, it's everyone's worst fears confirmed," said Andrew Linington of the seafarers union Nautilus International.

"Very little attention has been paid to waterborne attacks, where we believe vessels are still incredibly vulnerable, as pirates have demonstrated."

INTERTANKO, an association whose members own the majority of the world's tanker fleet, said it was "disturbed" by what increasingly appeared to be an external attack on the ship.

"Tanker owners naturally remain concerned at any development that could put crew, ship and cargo at risk, and thereby also threaten to interrupt the flow of free trade," it said.

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

Merchant ships have been increasingly plagued by Somali gangs operating in the Gulf of Aden and the Indian Ocean.

"While if it is shown that the attack on the M. Star was the result of a terrorist attack, there would certainly be a need for greater vigilance and perhaps a review of security protocols," said J. Peter Pham, a strategic adviser to U.S. and European governments.

"If the attack on the M. Star is not a one-time event, but others follow, it does place additional stress on both the shipping industry and the navies of the world."

Security analysts said an insurgent attack on a tanker based on al-Qaida's asymmetric warfare tactics could mean that a different front was opening up.

"Even if this attack was relatively low-level and caused limited damage, it clearly demonstrates the terrorist intent in these waters and against the oil and shipping sectors," said Metsa Rahimi, intelligence analyst with consultancy Janusian. "The threat certainly cannot be underestimated."

Security sources said that there have been arrests of suspected militants linked to plans to stage assaults on naval ships or merchant vessels on the high seas.

"What we will likely see is shippers returning to a more circumspect security posture with respect to allowing smaller vessels to approach, whether that's establishing an exclusion zone of 500 meters around every vessel or adopting nonlethal counter measures such as long-range acoustic devices," said Jonathan Wood, global issues analyst at Control Risks.

Insurance risk

The narrow Strait of Hormuz is a gateway to the oil-producing Gulf and handles 40% of the world's seaborne oil. Al-Qaida has threatened to attack shipping there in the past.

"An attack would cause greater concern and could have a larger impact on exporting countries in the Gulf, many of which have no export option except for shipping via the Strait of Hormuz," Mr. Wood said.

Fears of an attack have raised worries over insurance risks.

"Underwriters are monitoring the situation but apart from the obvious damage to the vessel, there is no definite information on which to base a view," said Neil Roberts, a senior technical executive with the Lloyd's Market Assn.

Analysts say any designation of Hormuz as a high-risk area by the LMA's Joint War Committee would be a major development.

"Given the significance of the waterway, no decision regarding this would be taken lightly," the LMA's Mr. Roberts said.

But Control Risks' Mr. Wood said it was unlikely that al-Qaida or affiliates could sustain a campaign involving attacking merchant shipping. "The attacks are generally more difficult to execute, and the required capabilities are relatively scarce."

Maritime experts said it is difficult to blow up or sink tankers, as they are built with watertight compartments able to withstand damage, as well as the modern fleet having double hulls. Trying to attack a moving tanker is also difficult.

"Just placing explosives in the vicinity of a ship is almost impossible to breach it unless you have significant quantities—you end up just denting it," said David Welch, managing director of Ramora UK Ltd., an explosive ordinance disposal and counterterrorism specialist.

"The double hull issue is a real nightmare from the terrorist's perspective because all the energy is lost in the transit between the outer and inner hull."