Security analysts say the ease with which Somali pirates have captured a huge range of vessels illustrates how much more at risk global shipping is to terrorist attack than the airline industry, which massively improved security after September 2001.

Al Qaeda has launched or planned several seaborne attacks in the past decade -- notably the suicide bombing of the USS Cole in 2000, which killed 17 American sailors, and a similar attack two years later on the French supertanker Limburg, which killed one crewman and spilled 90,000 barrels of oil into the Gulf of Aden.

"There had been an argument that terrorist groups don't have the nautical skills required to capture a ship. But the Somali experience shows you don't need a high degree of skills," said Ian Storey, a fellow at Singapore's Institute of Southeast Asian Studies and an expert on piracy. "It shows how easy it can be."

It's a lesson al Qaeda had already learned. After the Limburg bombing it issued a statement boasting that a major vessel had been crippled by a low-tech attack:

"If a boat which didn't even cost $1,000 managed to devastate an oil tanker ... imagine the extent of the danger that threatens the West's commercial lifeline, which is petroleum."

Peter Chalk, a terrorism-risk analyst at the Rand Corporation think tank, said in a report on piracy and terrorism in June that "there has been a modest yet highly discernable spike in high-profile terrorist incidents at sea over the past six years".

A captured ship could either be held for ransom to boost the coffers of terrorist groups or turned into a floating bomb that could devastate a port. Governments have been aware of the threat for years but analysts say little has been done to deal with it.

"Many of the vulnerabilities that have encouraged a higher rate of pirate attacks also apply to terrorism, including inadequate coastal surveillance, lax port security, a profusion of targets, the overwhelming dependence of maritime trade on passage through congested chokepoints ... and an increased tendency to staff vessels with skeleton crews," Chalk said.

"These gaps and weaknesses provide extremists with an opportunity to move, hide and strike in a manner that is not possible in a terrestrial theatre."

ECONOMIC RISK

Shipping stocks have already been battered by the impact of the global financial crisis on international trade and demand for commodities. Many are trading at distressed-asset levels.

A terrorist attack on shipping would push up insurance premiums and further pressure the industry. Fears of a floating bomb attack in the Strait of Malacca led Lloyd's to temporarily declare the area a war risk zone in 2005.

And an attack aimed at shutting down a major port such as Singapore or disrupting a key shipping lane like the Strait of Hormuz, through which 40 percent of the world's oil passes, could do real economic damage.

"Maritime attacks offer terrorists an alternate means of causing mass economic destabilisation," Chalk said.

"Disrupting the mechanics of the global 'just enough, just in time' cargo freight trading system could potentially trigger vast and cascading fiscal effects, especially if the operations of a major commercial port were curtailed."

There is debate on how effective a floating bomb would be.

"The jury is still out on the doomsday scenarios," Storey said. While some analysts have said ships carrying crude oil or liquefied natural gas (LNG) could be captured and detonated, Storey noted that crude is not very flammable and LNG carriers are robustly constructed and include significant safety features.

Ships carrying ammonium nitrate are a bigger concern -- the fertiliser is highly explosive when mixed with fuel oil and was used in the Oklahoma City and Bali bombings.

When a fire detonated around 2,300 tons of ammonium nitrate aboard a vessel in Texas City's port in 1947, the blast caused a 5-metre tidal wave that swept through the town. At least 567 people were killed and more than 5,000 injured. Hundreds of homes were destroyed and two small planes were blown out of the sky.

It was the worst industrial accident in US history.

Analysts have long warned of the danger that pirate groups and terrorist organisations could join forces, increasing the risk of major seaborne attacks. The al Qaeda-linked Abu Sayyaf group killed more than 100 people in the bombing of a ferry in Manila Bay in 2004, the deadliest ever militant maritime attack.

But Chalk and Storey both said fears of a tactical nexus emerging between pirates and extremist groups were overstated.