By Justin Webb BBC Washington correspondent

The Iraqis are not the only ones calling on President Bush to "put up or shut up".

There have been calls from American politicians for President Bush to meet the challenge laid down by Iraq and provide some information to contradict Saddam Hussein's claim to be free of weapons of mass destruction.

The White House knows only too well that the ball is back in its court.

But administration officials are suggesting now that the solid evidence they say they have is not contained in one single photograph or declaration from a defector.

No 'smoking missile'

A senior administration official is reported to have compared the situation with attempts to define pornography.

"You think you know it when you see it but two people might disagree about a precise definition."

	:	Military build up
Map showing key US regional bases

Other administration officials look back in history to the Cuba Missile Crisis and say people waiting for a similar approach this time will be disappointed.

In other words, they cannot do today what the Americans did in dramatic style 40 years ago when they produced photographic evidence of Soviet missiles in Cuba and thus proved the Soviets had been lying.

Instead, says the White House, the intelligence picture of Saddam's weapons stocks is made up of a pattern of tell-tale signs leading to only one reasonable conclusion.

There is no "smoking missile" but there are - say the Americans - plenty of pieces of circumstantial evidence.

Hard sell

The trouble is that even that evidence might not be capable of being revealed without at the same time showing how much intelligence the US has about Iraq and what the sources of that intelligence might be.

The CIA will be telling the president to be cautious if he wants to know more in the future.

	Iraq's dossier Contains 12,000 pages in Arabic and English covering Iraq's chemical, biological and nuclear capabilities. 2,100 page nuclear component being studied by IAEA in Vienna. Declaration being examined first by five nuclear powers on Security Council. See also: First failure? Resolution 1441

The likeliest short-term outcome is that the Americans will share some information with the arms inspectors to try to improve their ability to do their job.

That will be fine for a few weeks, but as the short term becomes the medium term the Americans will have to make a judgement.

Assuming that they are not convinced by the Iraqi document (a safe assumption) and assuming the arms inspectors find nothing new, they will have to decide at some point to go back to the Security Council and try to persuade other members to back a war.

At the moment it looks as if the evidence the Americans provide in support of their view will be similar to the evidence they have provided so far to the world community.

It might not be enough to convince some nations - the Russians and the French in particular - that the war is necessary.

The president has to make some delicate judgements.

In the end, he might well decide that the simplest path is the lonely one that some of his advisers always preferred: that America simply does the job itself.