US military ground and air forces in Afghanistan have so far taken centre stage in the war against terrorism. But beyond the headline-grabbing land campaign, a much broader effort is underway at sea, reflecting growing concerns about the infiltration of the murky world of global shipping by the al-Qaeda network.

In the most overt sign of international cooperation in the war on terrorism seen so far, some 90 warships from countries including the UK, Germany, France, Australia, Italy, Japan and Bahrain - under the command of the US Fifth Fleet - are patrolling the waters around the Arabian Peninsula and off the coasts of Pakistan and east Africa.

One of the main tasks of this huge armada is to cut off the most likely escape routes for al-Qaeda members attempting to flee Afghanistan and gather intelligence on the maritime-based logistic network believed to have been developed by the terrorists.

This display of naval firepower is concentrated on just one facet of al-Qaeda's use of the shipping industry. Western intelligence agencies suspect the maritime capabilities of the terrorist organisation extend much further.

There is strong evidence to support claims that Osama bin Laden and his associates have built up a dedicated al-Qaeda fleet of small freighters, operating on the fringes of the industry. But it is not the vessels under direct control of al-Qaeda which authorities consider pose the greatest risk to international security.

There is widespread concern that an organisation capable of the suicide hijackings of airliners could readily adapt these capabilities to major shipping targets.

The entire shipping industry went onto high alert following the hijackings of four US commercial jets in September. In the worst case scenario, terrorists could load a nuclear device, most likely a "dirty bomb" made of nuclear material wrapped around conventional explosives, onto a cargo ship and detonate it in a major port.

These concerns were illustrated in December when British anti-terrorist specialists intercepted and boarded the Nisha, an Indian-owned bulk carrier, in the English Channel. It was bound for sugar refinery in Silvertown, near London's Canary Wharf financial district.

The authorities claimed to be acting on a tip that the ship was carrying "terrorist materials" but three days of searching found nothing and it was allowed to dock at the Thames terminal in early January.

After the events of September 11, doomsday scenarios like these cannot be discounted but maritime security experts believe the biggest threat comes not from terrorists taking direct control of ships, but from adapting a standard shipping container as a weapon.

The scale of the problem is immense, with around 35m of these boxes estimated to be in use around the world. Organised crime has for years abused loopholes in the loose regulatory system, which makes it both impossible to track the movements of this boxes or to verify the contents.

"The container is a Pandora's Box," said one senior US shipping official. "The threat is what has happened to the box before it goes on to the ship . . . We have to be able to detect whether the box has ended up in the wrong hands."

Maritime security experts agree that the current system, which allows containers to move around the globe almost completely unhindered will need to change. Some 13m containers enter the US every year and although inspections have been stepped up only about 2 per cent are physically inspected.

"The first time a container is used as a weapon we will start wondering what is in all the other boxes," said Stephen Flynn, a commander in the US Coastguard and a senior fellow at the Council of Foreign Relations in New York.

The US is so concerned about this threat that will bring the issue up at the G8 summit in June and has already put forward proposals to counter terrorism in the industry through the United Nations' International Maritime Organisation.

Shipping containers do not only make potential weapons. Organised crime syndicates specialising in human trafficking use purpose-built containers designed to carry around 20 people hidden behind "false cargo".

The extent to which al-Qaeda could use this method to move personnel around the world became apparent last October, when Italian police found an Egyptian national inside a shipping container fitted out as a makeshift home.

Rizik Amid Farid, 43, was discovered during a routine inspection at the transhipment hub of Gioia Tauro, southern Italy, of a ship en route from Egypt to Toronto.

He was held by the authorities as a suspected al-Qaeda terrorist after being found with items including a laptop, two mobile phones, cameras, a Canadian passport and a certificate saying he was an aircraft mechanic.

It was this type of discovery that prompted the Pentagon to launch the "Leadership Interdiction Operation" (LIO) in the Indian Ocean last November, bringing together an array of seapower from across the globe.

The Bahrain-based US Fifth Fleet, whose area of command stretches across 7.5m square miles from Kazakhstan to Somalia, warned all shipping, particularly those operating off the coast of Pakistan, that they could be intercepted, boarded or destroyed in search for bin Laden and his network.

By early February, US security forces had boarded just eight vessels under LIO and found nothing as struggle to separate criminal from terrorist activity. One UK defence source said: "The scale of smuggling in the Indian Ocean is much greater than originally believed with organised crime playing a major part."

The lack of success by coalition naval forces in apprehending any terrorist suspects underscores the scale of the task facing the authorities in tracing Al-Qaeda's links to shipping.

The secrecy that pervades the industry, making it so attractive to criminals has proven equally alluring to terrorist networks and so frustrating to investigators.

The main obstacles to transparency is the convoluted ownership structure, commonplace throughout the industry, and the lack of effective regulation in many parts of the world.

In many cases a ship is registered with a company which is simply a brass plate on a door in an offshore domicile. Ships can also be registered under so-called "flags of convenience," in countries such as Belize, Honduras, Liberia and Panama.

The International Transport Workers' Federation (ITF) allege these registers have lax standards of safety and training and no restrictions on the nationality of the crew.

In many cases ships are simply not registered or registered using false information. It is also possible for vessels to "disappear" from a register altogether.

These vessels may find it difficult to enter ports in the developed world, they are unlikely to be prevented from calling at smaller ports in parts of the world with less stringent controls.

David Cockroft, secretary general of the International Transport Federation, says: "The whole shipping business is based on false identities. Shipowners head the list using shell companies but false identities and qualifications are part of the mix too."

There are real fears that terrorists could easily pose as seafarers by obtaining false documentation and rudimentary maritime training.

Mr Cockroft claims he obtained a first officer's certificate issued by Panamanian authorities for $4,000, despite having no maritime experience.

It is in this netherworld that al-Qaeda's fleet of vessels is thought to operate. US naval sources confirm the existence of a list of vessels but insists it is "fluid" and updated regularly.

Industry insiders believe the fleet could be anywhere between 10 to 80 vessels strong made up chiefly of small freighters below 1,000t.

US investigators have uncovered clear evidence of that al-Qaeda operatives were buying vessels seven years ago.

Wahid El-Hage, who was sentenced to life imprisonment on terrorism conspiracy charges in 2001 for his role in Al-Qaeda's bombing of the US embassies in East Africa, bought a tramp freighter, named Jennifer in April 1994.

The ship appears to have continued carrying legitimate cargoes around the Red Sea before it reportedly sank off the Omani coast two years ago.

The wreck of the ship, by then called Sky 1, was never found.