Nato ponders how to turn words into action: Christopher Bellamy, Defence Correspondent, calculates the number of troops needed to create 'safe havens' in Bosnia

THE BRITISH component of the Nato ACE mobile force - the personal force of Nato's Supreme Commander in Europe - was last night on one week's notice to move, its usual state of alert. As the Supreme Commander's personal reserve, the ACE mobile force would probably be the first deployed by Nato on behalf of the UN if, as looks increasingly likely, more military forces are sent to secure supply lines into Bosnia.

United Nations approval of the use of force is said to be imminent but it is not yet clear who would provide the forces or take command of any military operation. The nine-nation Western European Union (WEU) may have the leading role within Nato if the US is unwilling to provide ground forces. Nato's International Military Staffs (IMS) - the organisation's top planning cell - has been examining three options in increasing order of gravity: maritime sanctions, providing observers and humanitarian assistance and, finally, protected corridors into Bosnia.

The most extreme option, demanded by Baroness Thatcher - air attacks on targets in Serbia - is not contemplated at this stage. High-precision attacks of this kind would ideally be carried out by the US although the RAF now has the technical means for them.

Following the IMS's recommendations, Nato's political committee will discuss contingency plans at an emergency meeting on Friday. The meeting of ambassadors from the 16 Nato members will discuss various options drawn up by the IMS but will not take any decision on whether to put them into effect. One source said the meeting would give Nato military authorities further directions but would not authorise any particular plan. This might take several days more. Then the first forces would start moving.

Nato's Supreme Allied Commander (SACEUR) is General John Shalikashvili, the US general who commanded Operation Provide Comfort to deliver aid and provide security for the Kurds in northern Iraq last year. Gen Shalikashvili's appointment is not merely coincidence. With a fairly modest presence on the ground and air patrols overhead, the Iraqis kept clear of the Kurdish enclaves. However, they had just been soundly and spectacularly defeated by the allies in the Gulf war. The situation in Bosnia is more complicated.

With Britain one of the three UN Security Council members to table the resolution, the political pressure for Britain to provide troops is strong, although gloom and reluctance prevailed in the Ministry of Defence yesterday. MoD sources insisted that until they were given detailed tasks it was impossible to plan in more than a general sense.

The options for Nato or the WEU, acting on behalf of the UN, include the opening of a land corridor from the Adriatic to the beleaguered city of Sarajevo for armed relief convoys. Alliance planners think this might need up to 100,000 troops. Two divisions at least (up to 50,000 troops) would be needed to secure the area around Sarajevo and keep Serbian patrols and heavy weapons out of range, with perhaps two more to secure the supply route and outlying Muslim areas such as Gorazde and Zenica .

Such areas would become, in effect, 'safe havens' for Bosnian Muslims and Croats. Muslims and Croats outside those areas might try to reach them. Then there are the numerous detention camps scattered throughout Bosnia which must also be inspected and the inmates fed. 'They'll all be pretty isolated and if you put troops there to guard them they'll have to be supplied,' said a military source. The preliminary Nato military estimates say at least 12,000 troops would be needed to hold Sarajevo airport alone.

The RAF was also tight-lipped. It said that until given an exact mission, it could not identify numbers or types of aircraft, although the most likely requirement would be for early warning or reconnaissance aircraft. Two British E3 Sentry Awacs early- warning aircraft have been flying over the area but have returned to Britain while Nato aircraft take their turn.

The ACE mobile force is a brigade - about 5,000 troops including three infantry battalions - currently commanded by a Belgian. Britain contributes one infantry battalion, (about 600 troops), currently the Duke of Wellington's Regiment, one artillery headquarters, a reconnaissance squadron, engineers and signals and a large logistics support battalion (about 1,000 troops). Other troops come from Germany, Denmark, the Netherlands and Italy.

Should Britain decide to send ground troops, the available, relatively light, forces are 3 Commando Brigade based at Plymouth, 5 Airborne Brigade based at Aldershot, including the Parachute Regiment, 19 Infantry Brigade based at Colchester or 24 Airmobile Brigade, based at Catterick.

If the task is defending supply routes, 19 Infantry Brigade has Saxon wheeled armoured personnel carriers (APCs), comparable with the APCs already deployed in Bosnia. If heavier forces are required, they would have to come from Germany, although army sources believe the massive Challenger tanks and Warrior infantry fighting vehicles would be too big and heavy for the narrow mountain roads and bridges.