Over recent days a series of incidents in Yugoslavia - apparently caused by Nato bombs or missiles going astray - have left dozens of civilians killed or injured.

Nato clearly regrets such episodes. But their increasing frequency raises questions about how precision-guided weapons can go astray.

Is Nato really living up to its claim that it is mounting the most accurate air campaign in history?

Civilian casualties in Yugoslavia are bad news for Nato and are inevitably seized upon by the government in Belgrade, which is well aware of the unease in several Nato countries about the way the air campaign is going.

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Nato knows that it cannot prosecute this air war without risking casualties on the ground.

Its aim is to reduce them to a minimum. And it insists that Yugoslavia's transport infrastructure - especially bridges - is a legitimate military target.

But Nato clearly has nothing to gain from hitting civilian buildings in towns like Surdulica or Novi Pazar - locations where Nato weapons appear to have gone badly astray.

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Military experts would agree that Nato is waging the most accurate air campaign in history.

Nearly all the weapons being used are precision-guided and the rules of engagement for Nato pilots are strict; they are ordered to break off attacks if obvious civilian casualties might result.

But modern weaponry is not infallible. Simple map-reading errors in difficult terrain may well allow weapons to go astray.

Earlier on Tuesday, Nato bombs were dropped on Albanian territory while other Nato aircraft were bombing Yugoslav positions in Kosovo close to the Albanian border.

Quite apart from human error, the guidance systems of bombs may fail to work correctly or weapons may not come off the aircraft cleanly.

A bomb that falls merely seconds after its intended moment of release - perhaps as the aircraft manoeuvres sharply - is clearly going to land a good way away from its target.

In war - even in high-tech war - civilian casualties cannot be avoided altogether. The greater number of incidents in recent days is probably a reflection of the significant increase in the number of sorties flown.

The problem for Nato is in many ways one of presentation. In stark military terms the number of civilian deaths is still small compared with the scale of the air operation and the range of targets hit.

But in insisting that it is not at war with the Yugoslav people, Nato has prepared its own publics for a war without civilian deaths - a war which it is simply impossible to wage.