USS Abraham provides some air support to Afghanistan

When British troops in Helmand need air support, it doesn't only come from airfields within Afghanistan itself - it may come from a floating runway 500 miles away.

That runway is a US Navy aircraft carrier in the Arabian Sea, currently the massive USS Abraham Lincoln.

Over the last few months its 65 aircraft have undertaken up to 30 missions a day.

"Over 75% of the close air support missions we have flown from the carrier in the past three months have been in direct support of the UK Marines and forces in Helmand province," says Rear Adm Mark Guadagnini, commander of Carrier Strike Group Nine, the small armada of US warships operating in the region.

The USS Abraham Lincoln is the centrepiece of the fleet.

"We are the mobile artillery for the coalition forces," says Adm Guadagnini.

Spotting roadside bombs

Lt Sarah Abbott, an F-18 pilot, described one air strike, with a laser-guided bomb.

"I was engaged with the J-TAC, the guy on the ground who is the air controller. And we were in support of a troop in contact situation," she recalls.

"They were taking fire from an insurgent position, and we were called in to take out that insurgent position."

This is a classic case of air support for ground troops. But a surprising number of people on the USS Abraham Lincoln emphasise the care with which destructive force is now used.

This follows devastating mistakes by US and other Nato forces, in which Afghan civilians died, with deeply damaging consequences for the Western effort.

"We've got very specific sets of rules of engagement procedures and we spent months prior to getting here studying these," Lt Abbott says.

"Dropping bombs is not our measure of success," says another F-18 pilot, Lt Cdr Eric Taylor. The mission, he says, is to support the ground troops, to let them know the air power is available, and to use weapons only when called upon.

Other tactics include loud, low "show-of-force" passes over enemy positions to encourage them to keep their heads down.

The F-18s also use their sensors to try to locate possible improvised explosive device (IED) or roadside bomb positions.

Prowlers

The USS Abraham Lincoln does not only carry fighter aircraft. It is an entire air force at sea.

Its E-2 Hawkeye radar warning and control planes help co-ordinate the air campaign, including other coalition aircraft and drones.

And then there is the ship's detachment of EA-6B Prowlers - electronic jamming aircraft. They also have a role in countering the IED threat, but just how remains murky.

"I must be a bit careful with this one, because it gets classified fairly quickly," says Lt Cdr Thomas Huerter, the commander of the Lincoln's Prowler detachment.

The planes have the ability "to influence the electromagnetic spectrum", he says rather elusively.

"That gets into a lot of operational capabilities that I can't really get into. But the counter-IED threat is an enormous effort across the force, and just about everyone has a piece of it, and - sure - we have a piece of it."

Fortress at sea

Aboard the Lincoln, Afghanistan feels very remote. And it is.

The pilots endure seven-hour missions to and from the operating areas, with air-to-air refuelling to extend their planes' ranges.

The UK may have given up on aircraft carriers for the rest of this decade as a result of the recent Strategic Defence and Security Review, but it is clear that the US Navy sees the carrier as adding a critical element to the coalition effort - including flexibility.

"We can operate day, night, in bad weather, 24-hours-a-day if necessary. We bring the logistics capability to be able to surge for a long time," says Adm Guadagnini.

"Something can happen to any of the runways anywhere in theatre, so they might be prevented from operating, but we can still operate in international waters wherever we want to go."

That flexibility means the USS Abraham Lincoln has also operated in the Gulf, supporting US forces in Iraq. And does the commanding officer of the ship itself, Captain John Alexander, worry about Iran?

"We're always cognisant of what's going on on the coast of Iran, what's going on on the coast of Oman, and we try to keep track of where everybody is at all times," Captain Alexander says. "It would be imprudent of us not to do that. But I wouldn't say I worry about it."

And despite talk of China, for example, developing new missiles that could threaten US carriers, Captain Alexander says he believes the carrier will remain a potent symbol of the US ability to project influence. "I see it fulfilling that role far into the future," he says.

Certainly the impression aboard is that the USS Abraham Lincoln really is like a fortress at sea. And by the end of its deployment, which is approaching, its planes will have completed some 2,000 missions over Afghanistan.