By Jonathan Marcus BBC defence correspondent in Doha

While there is growing speculation that Saudi Arabia may allow the Americans to use at least some of its bases in a UN-sanctioned military operation against Iraq, the United States Defence Department is still preparing for all eventualities.

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In a few weeks' time, elements of the US central command (Centcom) will deploy to the al-Udeid base in the small Gulf state of Qatar.

Located some 30 minutes' drive from the capital, Doha, the base could play a central role if there is military action against Iraq.

The base has the longest runway in the Gulf.

Through the heat haze you can see the silhouettes of shelters and US military aircraft, tankers and transports lined up on the concrete.

But it is what you cannot see that makes al-Udeid so important.

Over the past year an air combat command centre has been installed here, capable of controlling the air war against Iraq should a similar facility in Saudi Arabia be unavailable for political reasons.

Close to the action

The Centcom deployment is just routine, according to the Pentagon's official line.

But it is a step that will bring the key operational headquarters for any invasion of Iraq much closer to what the military call the "theatre of operations".

Central Command has been running Afghanistan operations

Britain too could participate in this exercise.

A small team from the Ministry of Defence in London is already here in Qatar examining the options.

Qatar's official position is that it has not yet been asked by the Americans if its facilities can be used.

Its government favours a diplomatic solution, but commentators here have few doubts.

If the base at al-Udeid is needed for a war against Iraq, it is highly unlikely that the authorities here will refuse.