Bosnia: Navy treads fine line between training and the real thing: On board HMS Invincible, Christopher Bellamy watches preparations for possible air strikes

'DANGER - armed aircraft' proclaims the sign on Invincible's flightdeck. Beyond, there are four of the ship's six Sea Harriers, with live Sidewinder air-to-air missiles and practice bombs.

Yesterday 12 sorties were planned - six pairs of Sea Harriers on training flights. But this is an operational area: they are carrying live missiles, they are flying over real Bosnian terrain and taking real photographs. The distinction between training and operational flying is a fine one.

With French United Nations troops intermingled with the Serbs on Mount Igman, overlooking Sarajevo, and the tantalising hope that the Geneva peace talks will at last be successful, the UN looks unlikely to ask Nato to launch air strikes immediately. 'It's a customer-supplier relationship,' said Invincible's captain, Captain Fabian Malbon. 'The UN is the customer and Nato is the supplier.' On Monday night Nato agreed to provide close air support in Bosnia if requested to by the UN.

The 'supplier' has been gathering copious quantities of information, air photographs such as the 8in x 10in prints analysed in Invincible's briefing room. But the UN - the customer - is not supposed to gather 'intelligence'. So the photographs are filed at the headquarters of the fifth Allied Tactical Airforce in Vicenza, waiting until that transaction takes place.

Invincible's Sea Harriers can do three jobs: air-to-air combat, close air support - attacking ground targets - and photo-reconnaissance. In the last couple of weeks attention has switched to bombing and reconnaissance. If Nato is asked to attack targets there will be 'slots' for the aircraft from the US carrier Theodore Roosevelt, which was off Corfu yesterday, for the RAF's Jaguars and the Navy's Sea Harriers.

The Sea Harriers are uniquely flexible, as they have reconnaissance cameras permanently fitted, whereas the RAF's Jaguars have to have special pods fitted instead of bombs.

The air operation is codenamed Deny Flight, a name suggested by the enforcement of the air exclusion zone over Bosnia-Herzegovina. But now Deny Flight has been expanded to include the possible use of air strikes in support of UN Resolution 836.

The line between training and operations is most blurred where reconnaissance is concerned. If they are good photographs, they will not be thrown away.

This is a new theatre for the pilots of 800 Naval Air Squadron. They have been practising bombing on the ranges in southern Italy, and spying out the land in Bosnia.

They have been working out how best to take the photographs - from what height, at what angle. The weather has been particularly bad in the past few days. 'Recently we've been concentrating on practising close air support,' said Lieutenant Commander Chris Neave, the commanding officer of 800 Squadron.

'The terrain is hard for anybody - very mountainous. I haven't any experience of how bad the weather will get out there, in the winter.'

You do not get the feeling that the Harriers are all bombed-up with nowhere to go. Getting to grips with the new role - ground attack - and the new theatre of operations keeps everyone on Invincible fully occupied.

And the aircraft are operating over Bosnia, where they could be fired on, which adds an edge to the 'training'.

Yesterday most of Invincible's crew hoped the peace negotiations which have failed so often would succeed this time and that they would not have to bomb real targets.

But if they do, the planes can deliver laser-guided bombs, 'dumb' bombs or cannon fire. The crew are uncertain what will happen but if Nato requests air support they will get about 24 hours' notice before the first strike.

That will send a frisson through everyone: no matter how well-practised they are, something would be wrong if it did not.

Further south, a flotilla of ships is blockading the Montenegrin coast - Operation Sharp Guard, to prevent supplies reaching Serbia and Montenegro. In the central Adriatic, Invincible has little to fear. But off the Montenegrin coast, within range of missiles similar to the Silkworms used by Iraq in the Gulf war, the crews wear protective flash-gear at all times. What remains of the former Yugoslav navy is not much of a threat but a potential threat and, as in all naval and military operations, never discounted.