Four years since the fighting stopped in Bosnia, international troops and massive injections of foreign aid have still brought only a very fragile peace.

On the bank of the river that runs through Mostar, the people are trying to restore the ancient bridge, blown up by Croats six years ago, to its former glory.

The Deputy Mayor, Safed Aruchevic, says only then can Mostar put the war behind it.

Bridget Kendall reports from Mostar on the reconstrucion of the Balkans

"This bridge will become the symbol of the whole of Bosnia-Herzegovina. I have the agreement of all sides to that: the Serbs, the Muslims and the Croats," he said.

But it will take at least two more years - and more than a bridge to reunite Mostar.

A city divided

On the eastern side of Mostar, the Muslim call to prayer echoes among shattered buildings. But cross into west Mostar, where the Croats live, and you'll find few Bosnian Muslims.

Four years on, the town still looks like a war zone

War-torn Mostar is still split down the middle, with separate police forces, schools, football teams - even, until recently, different currencies.

And in the middle - as Chris Riley, local spokesman for the international administrator, showed me - a bombed-out no-man's-land of derelict ruins.

This is the dividing-line between east and west Mostar. It's a central square, which was the scene of fairly intense fighting during the 1993-1994 war between the Muslims and the Croats.

Because it is right on the dividing-line, there is less incentive from either side to rebuild or repopulate it, says Mr Riley.

Journalism compromised

Even the airwaves are divided. In west Mostar, the local station insists it is broadcasting in Croat. Yet in east Mostar, at a Bosnian Muslim radio station, I was told the same language, with minor variations, was pure Bosnian.

And journalists say they can't possibly report on the whole city. Crossing the divide is too dangerous.

"I can't take my car and go there and wait there. Because some fool might smash my car and smash me also. So, for security reasons, I can't go there and have both sides of the story," said journalist Misad Beram.

Relying on the international police

No wonder, in this atmosphere, the international escort troops in Mostar are still very much in evidence.

Armoured personnel carriers rumble through a land that still looks like a war zone: House after house empty, shells without either roof or window.

Emerging from what looks like an abandoned building, one solitary Serb greets the soldiers. Evicted from his village during the war, he's only returned to camp in his old house because of the twice-daily visits by Nato.

"While the international troops keep an eye on us we feel safe," he said.

"But if they leave, there might be more threats and trouble."

Like much of Bosnia, the peace in Mostar is paper-thin, it seems.

Even Luis Nunez Martinez, the general in charge of civilian relations, admits that four years on Nato troops here are still essential.

"By now, we are bringing them a kind of stability. But if we were not here, probably they would start again with a war," he said.

In the old city of Mostar, two workmen, perched precariously on a roof, hammer down tiles.

But it will take more than restored buildings to bring back the tourists to these cobbled streets.

The lesson of Bosnia, for the leaders gathering this week in Sarajevo, is that it takes years, not just to erase the scars of war, but to get opposing sides to trust each other.

And until then, whether Nato and its allies like it or not, stability in the Balkans is their responsibility.