After a meeting of Nato foreign ministers in Brussels on Monday, the alliance appears to be edging closer to accepting the idea of sending ground troops to fight their way into Kosovo.

From the start of the air campaign on 24 March, Nato's political leaders and top generals have insisted that the air bombardment of targets in Yugoslavia was the best means of striking at President Milosovic's military machine and achieving the alliance's goals in Kosovo.

BBC's European Affairs Analyst William Horsley

Ground troops would only be sent in, they have said, when there was a "permissive environment".

That, it was explained, meant after the complete end of hostilities, the withdrawal of all Yugoslav forces, and a signed agreement from President Milosovic that he would allow Kosovo Albanian refugees to return in peace, protected by a Nato-led international force.

After nearly three weeks of allied bombing, that kind of "permissive environment" still looks like a distant goal.

General Sir Charles Guthrie, the British Chief of the Defence Staff, said on Monday that Yugoslav air defences "remain a threat to our aircraft and their pilots".

In all, within Kosovo, Nato claims to have hit no more than three convoys or depots of Serbian military vehicles.

Nato wants a "permissive environment" in Kosovo before sending in soldiers

Serbia is still thought to have over 300 tanks and 40,000 troops in the territory, ready to oppose any possible land invasion.

Nato's military planners appear to have calculated when the air campaign began that the "permissive environment" they wanted might be achieved by intensive air attacks, by both high-level bombers and low-level ground attack jets.

The idea was that this assault would paralyse the Serb military's transport, communications and supply of ammunition, and that low-flying fighter-bombers would batter the tanks, artillery pieces and troop units so badly that the Serb forces would be disabled as an effective fighting force.

Then, Nato forces would be able to occupy Kosovo without real resistance.

Limited impact

In reality persistently bad weather and the difficulty of locating precise targets have severely limited the impact of the Nato air attacks over Kosovo.

But the ferocious, nightly attacks on strategic and infrastructure targets across Yugoslavia as a whole have seriously damaged the capabilities of the Yugoslav military.

Now the US Government says it has long had contingency plans to send in ground forces, even without a ceasefire, and these are being updated.

So Nato appears now to be re-defining what it means by a "permissive environment", and there are probably two reasons why.

A successful invasion would require tens of thousands of troops

One is the acute risk of mass starvation among the hundreds of thousands of Kosovo Albanian refugees driven from their homes and trying to hide from Serbian forces in the mountains and woods of Kosovo.

Helping them would involve dropping food, blankets and medicines from the air, or deploying highly skilled Nato troops to protect them within "safe havens" in certain designated regions of Kosovo.

The other is the realisation that using air power alone, however intensively, may not be enough to destroy or drive out the Serb field forces in Kosovo, if they stay dug in and determined.

One more fundamental problem remains if NATO should decide that the environment was "permissive" enough.

It has no safe launching-place for a massive ground invasion, which it is thought would need tens of thousands of troops, at least, to be successful.

And it has no easy means of access, since an entry from either Albania or Macedonia would be through difficult terrain, and Serbia has mined many of those approaches.

So, just as Nato has begun to seriously consider sending ground forces into an enviroment in Kosovo that is not completely permissive, the prospects for making an unopposed entry are receding further - unless, that is, President Milosovic were to spring a surprise by suddenly suing for peace.