Lines on map sweep away Muslim villages: In the UNHCR office in Split, Robert Fisk found officials struggling to cope with the effects of Serbian tactics on Bosnian towns

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In the United Nations High Commissioner for Refugees headquarters in Split, the front lines move inexorably towards Tuzla, bulging a little towards the west then suddenly moving towards the city from the east. 'If you look at the military maps each day and compare them with the earlier maps, you can actually see the lines moving,' Jean-Paul Trumpf says. 'Looked at in isolation, they are just static diagrams. But with a whole set of maps you can see the advances being made. That's why we are finding whole new communities north of Tuzla under fire, being displaced, new people to feed and shelter.'

If only the maps would stay the same. If the armies of Bosnia, Croatia and Serbia would just stop moving the contours of their battles, life would be a lot simpler for the UNHCR's French logistics man in Split. Along with his colleagues in Metkovic and Belgrade, he has 82 trucks on the move up in the mountains of Bosnia, transporting 1,000 tons of food and blankets every day, 200 tons of it moving into Sarajevo alone. From the Croatian town of Metkovic, Mr Trumpf has six convoys a day humming up the highway to the Bosnian city of Mostar - the main road is now patrolled by troops of the Spanish legion - three of the convoys for Sarajevo, the others for Vitez and Zenitca.

Even the railway to Mostar has been reopened - up to a point. Two of the rail bridges in the city have been destroyed and the first goods train to move up from the port of Ploce had to stop short of the main station. Bosnia-Herzegovina never had time to take over its own railway network and the UNHCR has imported an Irish station-master, courtesy of Irish Rail's headquarters in Dublin, to sort out the rolling stock. But a solitary diesel locomotive, bedecked with UN flags, has already hauled the first relief supplies up from the port of Ploce to a stretch of track a few miles outside Mostar. The UN officials aboard carried special UN tickets; the Serbian gunners, whose artillery brackets the main highway as well as the railway line, were told the time of the train as a goodwill gesture.

Now the UNHCR has plans to put together a train on the other side of Mostar and ship goods to within 20 miles of Sarajevo. There is no time to repair the bridges. 'We would bring up the train from the coast and tranship the material on to trucks south of Mostar,' Mr Trumpf says. 'The lorries would unload it back on to the train on the other side of the city and the train would take it up towards Sarajevo.'

In one of his more sanguine moments, General Phillipe Morillon, the UN commander in Bosnia- Herzegovina, has suggested not only the repair of the bridges but the reopening of the rail track all the way into Sarajevo. Food and fuel by InterCity. Little wonder that the idea flies beyond reality. For in the room of the British liaison officer in the Split UNHCR headquarters are a series of maps, all showing the convergence of red arrows into central Bosnia. Red circles surround Sarajevo. Red arrows poke into Bosnian territory north-west of Livno, west of Bugojno, north-west of Travnik, north of Maglaj and east of Tuzla and Olovo. 'Heavy BSA (Bosnian Serb Army) artillery and infantry attacks on Bosnian positions at Gradacac and Brcko,' announced the British military assessment at the bottom right-hand corner. 'Heavy BSA attacks against Tesanj and Duboj . . . artillery attacks are reported on Mostar road in the area of Konjic . . . '

What happens if Tuzla is cut off? Or if the convoys into Sarajevo are halted? Or the airport remains closed? Or if the Serbs shell the railway line on both sides of Mostar? Already, UN military officers - both British and French - are planning for the worst contingencies. When the British this month escorted a UN convoy which had set out from Belgrade through the Serbian lines to Tuzla, they were in fact preparing for the sealing off of the Tuzla 'pocket' by Serbian forces. If the Serbs cut off the city and its hinterland from resupply out of southern Bosnia, the UN will simply bring in all food and fuel from the Serbian capital under the precedent which they have already established. The same applies to Sarajevo, which is already receiving food via UN convoys from Belgrade. Unhappily for the government in Sarajevo, this forward planning merely reflects the dismal military prospects which the UN holds out for the Bosnian Muslim forces.

According to Anne Shepherd, the British head of the UNHCR in Split, villages are now being displaced in the fighting north of Tuzla. 'We keep finding more people who are fleeing their homes and have nowhere to go,' she says. 'These are from new areas previously unaffected by the war. These people were not in our original plans.' The Mostar highway remains crucial to all UN planning. A hundred UNHCR drivers - only 15 of them from the former Yugoslavia - are currently working seven days a week on the main roads of Bosnia and the UN is now able to berth a ship in Ploce every week, bringing railway wagons onto the dockside to receive food and supplies.

But if the Mostar road comes under heavy shellfire - or is breached by the Serbian forces - then all convoys will have to travel north over two steep and slushy tracks. The more evil of these two roads pirouettes up through the Prenj mountains where the Royal Engineers have promised to push back the snow and ice throughout the worst of the winter. Most convoy drivers regard the track with a mixture of venom and fear. 'If we are reduced to resupply along the mountain roads, then we are going to be 48 hours underway with traffic jammed up in the snow in both directions,' an English aid driver commented. 'The Brits may keep it open but that doesn't mean we can move trucks on it.' He was not exaggerating. Even a convoy carrying the British Secretary of State for Defence along the track last week was held up by a traffic jam for almost half-an-hour.

The worst nightmares of the relief agencies, of course, have still to be realised. The much-trumpeted fear of 10,000 deaths by starvation and cold remain as mythical as the 100,000 troops who - according to anti-interventionists - would be needed to end the violence in Bosnia. But the UNHCR will be watching the maps as much as they do the snow-line as it creeps down the mountains north of Split.