UK Prime Minister Tony Blair has responded to critics who say the aim of the Nato operation against Yugoslavia is unclear.

Mr Blair says the purpose is crystal clear - it is "to curb Yugoslav President Slobodan Milosevic's ability to wage war on an innocent civilian population."

Yet the statement hides a subtle shift in Western objectives.

While Nato air strikes were still just a threat, their aim was to force President Milosevic to sign the Kosovo peace plan, and allow in Nato peacekeepers.

But as the State Department spokesman James Rubin conceded on Wednesday night, as soon as military action began the objectives changed.

The new aims were listed by President Clinton:

• To demonstrate the seriousness of Nato's purpose
  
• To deter an even bloodier offensive against innocent civilians in Kosovo
  
• If necessary, to seriously damage the Serbian military's capacity to harm the people of Kosovo.

Deterring and degrading

On the face of it, that suggests that a Serb ceasefire in Kosovo could be enough to end the conflict, even without Belgrade's signature on the peace plan.

That would leave nothing resolved, despite the tonnes of ordinance already dropped on Yugoslavia.

The military objective, to seriously degrade the Yugoslav military capacity, is even harder to quantify.

Many people have pointed out that it is not high-tech weaponry that is causing the suffering in Kosovo, but just men with guns - the hardest target for air strikes to obliterate.

Kosovo independence

Indeed the growing Serb offensive in Kosovo, and the mounting tension in neighbouring countries, appears to show that the air strikes are working against Western objectives, at least in the short term.

And in the longer term other goals are being sacrificed.

It is been a long term western aim to deter Kosovo from seeking independence because of fears that the Kosovo Albanians would seek to unite with Albanians in Macedonia and Albania itself.

Such a re-drawing of the map could have far-reaching consequences, yet every bomb dropped on the Serbs seems to bring it closer.

One other objective has been stressed by Nato leaders - the need to underline the credibility of the organisation as it approaches its 50th anniversary.

The critics argue that the organisation has talked itself into a corner.

It has put itself into a situation where maintaining its credibility has become a key objective.