By Paul Reynolds BBC News Online world affairs correspondent

When governments build a case for war, they point to the misdeeds of their enemy. It is a time to be especially careful of their claims.

The basis for the accusation is not always open to close scrutiny - yet is sometimes the hinge on which the issue turns.

The temptation for governments is to exaggerate the threat - though, of course, opponents of war can underestimate it.

Officials say they have information from prisoners captured in Afghanistan

The claims by the Bush administration of links between Iraq and al-Qaeda are unusually important and unusually hard to assess.

If true, they would lay a legal basis for unilateral action by the US against Iraq, under the self defence provisions of the United Nations Charter.

And they are almost impossible to prove untrue. But they can be narrowed down.

The claims, from National Security adviser Condoleezza Rice and Defense Secretary Donald Rumsfeld, are of top-level contacts between al-Qaeda and Iraq going back a decade.

Further, Ms Rice says that prisoners in Guantanamo Bay have said that Iraq had provided "some training to al-Qaeda in chemical weapons development".

Mr Bush himself has weighed in with the "what if" threat. What if Iraq teamed up with al-Qaeda or another terror group and gave it weapons of mass destruction?

It is the Armageddon argument. It has the convenience of not needing any actual evidence. The possibility, and the example of 11 September, is enough.

What is known about any links?

• The report from the Czech authorities that one of the 11 September hijackers, Mohammed Atta, had met an Iraqi agent in Prague in April 2001. This report intrigued the world for months but now the Czech President Vaclav Havel has reportedly told the Americans he doubts if it true. Havel's view removes one of the bigger columns in the edifice holding up the conspiracy theory.

• A suggestion in some circles that Ramzi Yousef, who bombed the World Trade Center in 1993, escaped from New York on a false passport provided by Iraqi intelligence. Byy its nature, this allegation is impossible to verify or disprove.

• A claim that al-Qaeda refugees from the war in Afghanistan have found refuge in Iraq. Some of this relates to a group called Ansar al Islam (Partisans of Islam) which has taken over a small area near the Iranian border. This part of Iraq, however, is in Kurdish hands and outside the direct control of the Iraqi Government. Iraq is said by defectors to have links to the group, but Iraq, of course, could make contacts in order to encourage it to fight the Kurds.

• The Christian Science Monitor, in an article reproduced by the US State Department, interviewed one Ansar activist Rafed Ibrahim Fatah, now in Kurdish hands, who spoke of meetings in earlier years between the group and al-Qaeda leaders, though not Osama Bin Laden himself. This however does not necessarily implicate the Iraqis.

• The Sunday Telegraph in London said that Rafed Fatah and a senior al-Qaeda operative captured in Morocco, Abu Zubair (known as The Bear), underwent training in Iraq and would feature in the British Government's dossier against Iraq. They were not mentioned in the report. Nor was any alleged link between al-Qaeda and Iraq.

One has to conclude that, in the absence of more evidence, the links are tenuous.

In addition, Saddam Hussein has not previously shown a fondness for Islamic extremists. He comes from a revolutionary, but secular socialist background.

It is true that he tries to get money to the families of suicide bombers in the West Bank and Gaza Strip but that has to do with his wish to glorify attacks on Israel.

It is also the case that his agents tried to assassinate President George Bush senior in Kuwait in 1993. Of Saddam Hussein's capacity for violence, there is no doubt.

But links to al-Qaeda would be highly dangerous, probably fatal for him, if it became known to the United States.

This does not mean that links do not, will not, or have never, existed.

The problem is - we just don't know. Maybe Ms Rice and Mr Rumsfeld will explain.

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