By Jonathan Marcus BBC defence correspondent

Turkey's appears to have withdrawn its offer to let the United States use its air bases in any war against Iraq - assistance that could be critical in any US effort to topple Saddam Hussein's regime.

Indeed a whole raft of regional allies may be needed.

Carrier-based jets are not enough for an air war on Iraq

So how are America's friends lining up?

Its often said that if the United States really wanted to invade Iraq and topple Saddam Hussein's regime it could do so on its own without the military assistance of any allies.

Up to a point that may be true, but allies would be needed, not to provide troops but to provide bases and facilities from which to launch operations.

Why Turkey matters

Real estate matters - especially in an air war. Concentrating combat power requires acres of concrete on which to stable, re-arm and re-fuel planes.

Carrier-based jets and long-range bombers flying from the continental United States or Diego Garcia in the Indian Ocean cannot generate sufficient sorties for an air war of this scale.

None of the Gulf states gave their unequivocal support to the US

Moreover the US war plan would be to attack Iraq from many directions - air bases in the region will be critical.

That - quite apart from geo-political considerations - is why Turkey matters to US war plans.

The question of price

So too do bases in Kuwait (where US forces are already exercising), Qatar, Saudi Arabia and elsewhere in the Gulf.

Have any of these countries given an unequivocal "yes" to their territory being used for an attack on Iraq? The answer is "no".

In the case of Saudi Arabia there is genuine uncertainty. Maybe it would allow only supporting flights - tankers for example - from its bases.

Kuwait and Qatar seem far less equivocal. And few analysts doubt that if it comes to a war their bases will play a key part.

The Americans also need land borders with Iraq from which to stage troops. Here too Turkey is important.

But whatever the public reluctance at this stage, if it really does come to conflict, few analysts believe that Turkey will refuse Washington's demands - the real question is what price it will seek in return.