The Patriot has undergone substantial changes since 1991

It is unclear if these preparations are in response to some specific intelligence threat from Baghdad or just uncertainty about the intentions of Iraqi leader Saddam Hussein.

The Americans may fear Iraq could try to exploit the distractions of the US presidential campaign and embark upon some military moves of its own.

Best defence

A Patriot battery consists of eight launchers, 64 missiles and their associated radars, but it is not clear if the full battery will be despatched to Israel or just part of it.

The Patriot was widely used in the Gulf War but is not credited with hitting any incoming Iraqi missiles.

Iraq claims to have no more of its Scud missiles

Nonetheless, the Americans say that software improvements and other changes now make it the only credible western weapon that can defend against ballistic missile attack.

The potential Patriot deployment underscores the close strategic relationship between the US and Israel but it also raises significant questions.

Paper tiger?

It is not clear how many missiles Iraq has - if any - that would be capable of reaching Israel.

Iraq says it has none of the long-range Scuds, but Richard Butler, the former head of Unscom - the UN body overseeing the destruction of Iraq's banned weapons programmes, disagrees.

And recent US and German intelligence reports claim there has been renewed missile building and testing in Iraq, though apparently of shorter-range systems that it is allowed to build.

Coming in the run-up to a closely fought presidential campaign, the proposed Patriot move could have some minor domestic impact within the US, demonstrating the Clinton-Gore administration's resolve in dealing with Iraq.

But it also underscores that successive administrations have tried to deal with the problem posed by the Baghdad regime without being able to come up with any definitive answers.