US President George W Bush has thanked Saudi Arabia for its co-operation in the campaign against terrorism.

The phone call to Crown Prince Abdullah bin Abdul Aziz, the kingdom's effective ruler due to King Fahd's illness, looked like a calculated effort to staunch rumours of friction between Washington and Riyadh.

According to the White House, the president could not disagree more with the suggestion that Saudi Arabia is not acting as a good partner of the United States.

Still, the two governments have struggled to paper over differences in a relationship that is awkward even at the best of times.

Riyadh support crucial

Mr Bush's move underlines the fact that Washington sees Riyadh as pivotal in holding together Arab and Muslim support for the US-led campaign.

More broadly, Saudi Arabia is crucial to the United States strategically because of its oil, while for the Saudi government, the United States is the ultimate guarantor of its security.

But the ruling al-Saud family is deeply and nervously preoccupied with concerns about its own grip on power.

'Not enough'

This, in the eyes of many American commentators, has meant that Saudi Arabia has been unable to help its key strategic ally in its hour of need.

Equally, it means that it has failed to face up to the potential internal threat that the brand of radicalism preached by Osama Bin Laden poses.

Many have complained that the Bush administration has been too ready to excuse Saudi Arabia's failure to offer more overt backing.

Saudi Arabia has its own issues of concern - not least US support for Israel.

Clearly, given the sensitivities involved, it suits both governments at the moment to preserve a relationship in which co-operation goes on in ways that are limited, and need not necessarily be acknowledged publicly.