Ever since the start of the Nato air campaign against the Yugoslav military there have been calls for Nato troops to move in on the ground.

Before the alliance air offensive began, Nato was preparing to deploy a peace implementation force in Kosovo, providing both sides signed the international peace agreement presented at the Rambouillet talks.

Significant Nato forces have already been sent to Macedonia in preparation for this mission. They include some 12,000 British, French and German troops, a full-scale corps headquarters, tanks, armoured vehicles and heavy artillery.

Click here to watch an assessment of Nato's options by the BBC's David Shukman.

Desperate calls for ground troops

With the collapse of the peace process this force is simply biding its time. Given the potential threat from Yugoslav artillery or air attacks it has nonetheless taken up defensive positions.

Nato has made it clear that any move against its forces will be met with a swift and decisive response.

But many military experts believe that air power alone cannot achieve Nato's objectives.

Representatives of the Kosovo Liberation Army have desperately called for Nato troops to move into Kosovo to stop the ethnic cleansing.

Political objections

But at the moment there is no suggestion that Nato will send in ground troops.

• For one thing there is simply no will within the alliance to be drawn into a ground war - what would in effect be an invasion into Kosovo.

• Yugoslav forces are relatively well-equipped and would be expected to resist strongly. The terrain and weather would mitigate some of Nato's advantages - not least the effectiveness of its air power.

• The Clinton administration is already having some problems in convincing a sceptical Congress and public opinion about the merits of the current air operation. It would be much harder to gain support for extensive ground operations.

  What goes for the United States goes for other Nato countries as well. And if the US isn't fully behind such a mission nobody else is going to become involved.

Practical problems

But the principal problems are practical.

Very large forces would be needed, especially if Nato casualties were to be reduced to a minimum.

They would require a spring-board from which to launch their operations and time to build up the necessary forces.

It is highly unlikely that a country like Macedonia would allow itself to be the base for an invasion of Kosovo.

The infrastructure in the region is basic or non-existent. A comparison with the war to liberate Kuwait shows the magnitude of the problem.

Macedonia and Albania are not like Saudi Arabia with huge airfields and modern ports just waiting for the arrival of Nato forces.

Time limitations

But time would be the greatest practical problem.

The Belgrade government has been preparing for this sweep through Kosovo for some weeks.

All its forces were ready to go - some operations were already beginning when the separate negotiating teams sat down at Rambouillet.

By the time Nato would be ready to act on the ground, the fate of the civilian population of Kosovo would have been sealed.

Air power 'only option'

Nato insists that air power is the only option and that its current strategy is having an impact upon the Yugoslav forces' ability to carry out their operations in Kosovo.

The problem is that the effect of air power is cumulative. It will take time for the Yugoslav military machine to be seriously damaged.

And time, it seems, for the Kosovo Albanians is running out.

At present the only circumstances in which Nato troops might go into Kosovo would be unopposed. That is if the Yugoslav Army and police units pulled out altogether as a result of the Nato attacks.

That may be a long way off. But Nato is ready to reinforce its troops in Macedonia at short notice.