By Jonathan Marcus BBC defence correspondent

On 5 February, US Secretary of State Colin Powell is due to present additional evidence to the United Nations that is said to prove that Iraq is thwarting the efforts of the weapons inspectors.

US press reports speak of the White House and Pentagon agonising over how much sensitive information can be declassified and put before a wider audience.

Hans Blix: Iraq must do more

Mr Powell's new "revelations" come in the wake of the chief UN weapons inspector Hans Blix's report to the Security Council on 27 January.

This was widely seen as being much more critical of Iraq than many had expected.

It raised all sorts of questions; some old ones about past weapons activities that have never been satisfactorily answered.

But it also raised a basic question about the whole attitude of the Iraqis to the substantive disarmament process.

Public opposition

A large part of public opinion in the West - especially in Britain and the United States - still remains sceptical.

There are continuing calls for evidence of the so-called "smoking gun" without which, many critics argue, war should not even be contemplated.

But in all likelihood there is no "smoking gun" and its absence does not prove one way or another that Iraq either has or has not got weapons of mass destruction.

Intelligence is rarely an open and shut case.

UN inspectors are still looking for evidence

The celebrated revelation by the US of photographs showing Soviet missiles in Cuba was a rare and perhaps exceptional case of a "smoking gun" being revealed to the world.

More often, intelligence is like putting together a jigsaw with only half the pieces scattered on the table. From that you have to make a judgement of what the overall picture looks like.

With Iraq, western intelligence agencies probably have a good idea about what is on Iraq's shopping list.

They probably also have an idea of what equipment and materials may actually be going into Iraq.

In other words, their intelligence on what can be gleaned from outside the country is reasonably good.

Difficult times

Where these things go to once they are dispersed inside Iraq is a very different matter.

Here you need defectors and interviews with scientists; many of whom may individually say little but whose testimony when cross-referenced may provide useful pointers as to what is going on.

The Americans could also have satellite and other data showing Iraqi efforts to thwart the current round of inspections.

That would certainly add weight to Mr Blix's existing assessment.

These are difficult times for western governments.

There has been worldwide opposition to aggression

The level of popular cynicism directed at both politics and politicians is great.

Among some sections of European opinion, America in general and Mr Bush in particular, excite almost irrational levels of antipathy.

Some government insiders wonder if any evidence will convince the doubters.

One of the problems is that there are really two issues here and they are linked into the rightly emotive question of peace or war.

Does Iraq still have weapons or the desire to acquire them?

But if it does, what should be done about it?

Denying that there is a problem is one approach.

But the great empty hole in this debate is some real alternative to war should Iraq be deemed to be circumventing the inspectors' efforts.