By Paul Reynolds BBC News Online world affairs correspondent

One of the most intriguing questions in the "war on terrorism" is whether there are contacts between Iraqi leader Saddam Hussein and Osama Bin Laden's al-Qaeda network.

Intelligence agencies are constantly looking for the "missing link".

The latest manifestation is a report in the Washington Post that Iraq might have given supplies of the highly dangerous nerve agent VX to Islamic extremists sympathetic to al-Qaeda.

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Iraq itself has called the report "ludicrous".

The Post said that Washington had received a "credible" report that such a transaction had taken place in late October or early November and that the VX was smuggled overland from Iraq into Turkey.

If this were so, it would justify President Bush's claim that Iraq has helped al-Qaeda sympathisers and would be evidence to support his warning that Iraq could at any time give its weapons of mass destruction to international terrorists.

The story follows a claim by Condoleezza Rice, the US National Security adviser, earlier this year that some al-Qaeda prisoners in Guantanamo Bay, Cuba, said they had been given "some training in chemical weapons development" by Iraq.

The CIA Director George Tenet made a similar assertion in a letter to Congress.

President 'vindicated'

The Post story has been picked up, as often happens, around the world and the headline "Saddam Link to Bin Laden" has appeared prominently in one British newspaper, the Daily Express.

One of Washington's leading hawks, William Kristol, editor of the Weekly Standard, has also weighed in. He says that the Post report means that "the president has been right in saying that the coming war to remove Saddam is part of the overall war on terrorism".

There are two problems with this. The first is that the original story, written by a reporter with good links to intelligence sources, is extremely cautious and does not in fact conclude that the "missing link" has been found.

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It quotes one official as saying that he feels the evidence is credible but another that it might simply refer to a hypothetical concern.

The other problem is that the group mentioned in the Post story as the possible recipient of the VX agent is called Asbat-al Ansar (Band of Partisans).

This is a small but quite well known organisation based in the Palestinian refugee camp of Ein al-Hilwah in Southern Lebanon.

It is a radical Islamic faction linked to a number of plots in Lebanon and Jordan.

But there has been nothing to link it to Iraq, which has traditionally been viewed with hostility by Islamists.

After all, Saddam Hussein repressed a Shia uprising after the Gulf War and comes from the secular, socialist tradition of Arab nationalism.

More confusion

The Post says the faction has "recently established an enclave in Northern Iraq" for which there has been no evidence before.

Indeed there is confusion as to what group might have received the VX.

Given the tenuous nature of the evidence itself, one cannot place much confidence in it at this stage

Some of the reports which are based on the Washington Post story - including the British newspaper with the definitive headline - name another faction with a similar name - Ansar al-Islam (Partisans of Islam).

Ansar al-Islam is based near the Iranian border in Northern Iraq outside the area of Iraqi Government control, though defectors have said that Iraq does have contacts with it.

This might be partly to encourage it to fight the Kurdish forces which control North Iraq and which hope for the overthrow of Saddam Hussein.

Indeed, there has been recent fighting in the area. Ansar al-Islam is reported to be partly made up of al-Qaeda fighters who fled from Afghanistan.

Inconclusive reports

So, even the name of the possible smuggler of the VX is not agreed. Given the tenuous nature of the evidence itself, one cannot place much confidence in it at this stage.

It is a classic example of how difficult it is to establish this "missing link".

There have been other inconclusive reports before, among them an alleged meeting in Prague between one of the 11 September hijackers, Mohammed Atta, and an Iraqi intelligence officer.

Such a meeting however is now discounted and even the Czech President Vaclav Havel is said to doubt that it happened.

The search goes on.