This will be a different kind of war, but it will begin with old-fashioned methods. Only a handful of people know when.

President George W. Bush has been at pains to warn of a long struggle after last week's terrorist attacks on New York and Washington.

"He has prepared Americans for a measured, calculated response," said Colonel Terence Taylor of the International Institute of Strategic Studies, a think-tank. "He's making clear that that there is no silver bullet, that it will not be like some whizz-bang American movie and then be all over."

Although there has been a debate within the US administration about whether to target other countries viewed as assisting terrorists - particularly Iraq - the initial focus of military action is expected to be Afghanistan.

But in one very practical way the clock is ticking for Operation Infinite Justice - the name given provisionally to the US effort, but likely to be changed because it may affect Muslim sensibilities. Within weeks it will be snowing in mountainous Afghanistan, making one of the world's most inhospitable terrains even more challenging.

So the pressure is on if - as defence experts believe - part of the plan is to insert "special forces" - crack troops with specialist skills - to smash terrorists' hide-outs and cut off their sources of assistance.

Massive air and seapower is being assembled in the Gulf and Indian Ocean region. Within two weeks, at least three US aircraft carriers and their accompanying battle groups of guided missile cruisers, destroyers and submarines - as well as the aircraft they carry - will be in the Gulf and Indian Ocean region.

Together with F-15 and F-16 fighters and other aircraft being sent to bases in the Gulf, the US will have some 400 aircraft in the region. B-2 and B-52 bombers can be dispatched from bases far away.

The build-up indicates the seriousness with which the likely US action should be taken. But it does not give a clue as to what precisely will be attempted - and that is part of the plan.

"Surprise is absolutely vital," Col Taylor said. Rear Admiral Richard Cobbold, director of the Royal United Services Institute, said the movements of forces were consistent with a "sensible long-term build-up" rather than suggesting imminent action. US military practice is to build up massive air superiority before attempting anything on the ground. There is no question of Afghanistan's Taliban regime contesting control of the skies.

But US commanders will want to make sure they can protect and extract at a moment's notice anybody on the ground. They do not want to risk repeats of losses such as those suffered in Iran in 1980 and Somalia in 1993.

Donald Rumsfeld, US defence secretary, said yesterday: "I'd like to disabuse people of trying to draw parallels between previous conflicts and this one. It's not useful. I think that it's in a direction that's not helpful and won't be the correct one."

Nevertheless, defence specialists expect the main elements to involve air power, including cruise missiles strikes and special forces.

The primary objective would be Osama bin Laden, the chief suspect for the US attacks, and his network inside Afghanistan. A likely secondary objective would be to topple the Taliban regime that has harboured him.

The aircraft carriers are likely to be the main platforms for the attacks. Pakistan has offered the use of bases but the Pentagon is likely to want to limit their use - and not only for political reasons. Charles Heyman, editor of Janes World Armies, said: "The logistics chain from Pakistan into Afghanistan would be much too long and would need thousands of troops to guard it."

Mr Heyman said that in time a forward operating base could be established within Afghanistan, though this would also require large numbers of troops to guard it. This would serve as a base to launch raids against terrorist hide-outs and Taliban military targets, with the aim of disabling the regime's military capabilities.

Access from central Asia would require the explicit agreement of Russia. One official, asked whether Washington's requests to Moscow would be limited to overflight rights, said much more was being asked for.

Good intelligence will be essential, especially in tracing elements of Mr bin Laden's network. Col Taylor believed it was difficult but possible, particularly with help promised by General Pervez Musharraf, Pakistan's president. "Somebody must know where these people are," Col Taylor said.

Additional reporting by David White

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