By Paul Reynolds BBC News Online world affairs correspondent

By asserting that Iraq is in "material breach" of United Nations Security Council resolution 1441, the United States is positioning itself to go to war.

It has always held that, in the end, it has the right to decide whether Iraq has complied with the terms of the resolution, whatever the Council itself might say.

Now it can play this card when it wants.

IRAQI MATERIAL UNACCOUNTED FOR

Nearly four tons of VX nerve agents Growth media for 20,000 litres of biological warfare agents 15,000 shells for use in biological warfare 6,000 chemical warfare bombs Nuclear information

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For the moment, it is choosing not to do so - perhaps because its position has weaknesses. For a start, the British allies have not yet come alongside.

The British Ambassador to the UN, Sir Jeremy Greenstock, said after the Security Council had met the chief weapons inspectors that a false declaration made by Iraq about its weapons was not enough for it to be found in material breach.

Iraq had, he said, to show a lack of co-operation with the inspectors as well - as laid down by paragraph four of the resolution, which says that "false statements or omissions... and failure at any time to... co-operate... shall constitute a further material breach."

"We will be faithful to the text of 1441," said the British diplomat.

The United States, however, appears to be going beyond the terms of the resolution by asserting a material breach on the basis of an alleged false declaration alone.

The French and Russians argued in negotiations leading up to the resolution that both a false statement and a failure to co-operate would be needed. That is why they insisted on the use of "and" in paragraph four and not "or".

Washington seems to be ignoring this distinction. Britain is not.

Absence of evidence

Another factor holding up US action is that the assessment about Iraqi omissions is based on the absence of evidence.

It is argued that Iraq has not explained what happened to material it was believed to have had in 1998 when the inspectors left.

Saying that someone has not proved they destroyed something is not as good as proving that they have it.

It would not convince large sections of world, or even American opinion.

Take one piece of "missing" material as an example - 1.5 tons of VX nerve agent.

The British and US governments say that this has not been accounted for.

For now, the inspections go on

But according to a report in January 1999 by the previous UN weapons inspection group Unscom, the Iraqis said that they had destroyed the VX by dumping it and UN scientists found traces at the site.

Iraq will now be asked to produce some evidence to support its claim - the technicians who dumped it, the dates, the documents.

The US State Department has also revealed the name of the African country from which Iraq has allegedly tried to obtain uranium - Niger. This was another test case for Iraq to explain its intentions, yet it has not it seems done so.

If evidence is withheld in this and other such cases, Britain might then adopt the same position as the United States. But that moment has not yet arrived.

The more immediate expectation is that the inspections regime will be toughened in order to test all this. It is likely to be extended from sites and buildings to people.

Iraqi scientists are the ones who really know what might have been hidden.

Taking up position

Another factor holding up American action is the simple one that not enough US troops are in position yet to wage a war.

The US secretary of state says that, if there is a war, it will be as "swift as possible".

That presupposes the use of overwhelming force, which was Mr Powell's trademark in the Gulf War. That force is not yet available.

So it is convenient for Washington to allow more time and to give Baghdad another chance, even though the American UN ambassador John Negroponte - a determined character who showed his steel in the Reagan administration - said that it had already "spurned its last opportunity to comply".

If Saddam Hussein explains all - if he can - even at this stage he could just get off the hook.

But his window of opportunity is getting narrower and narrower.