Barnaby Mason BBC Diplomatic Correspondent

As the administration of President George W Bush clears the diplomatic decks for military action against Iraq, it is making clear that it will not allow a new mission by United Nations weapons inspectors to get in the way.

The Secretary of State, Colin Powell, told a congressional committee that the United States would oppose any move to send the inspectors back to Iraq without a new Security Council resolution and new authority. That would be a recipe for failure, he said.

The US and Britain want the new resolution to set up a tough and intrusive regime for inspections, spelling out the consequences of any obstruction or prevarication by President Saddam Hussein.

As one British official put it, there should be an urgent timetable and an early test of whether the Iraqi offer to co-operate was genuine.

No deadline

The key point for Washington is not to let the inspection process stretch out over several months, so that by the time it comes to any conclusion the cool winter months most suited to military action have passed.

Arms inspector Hans Blix will meet Iraqi officials on 30 September

But the Security Council resolution that set up the present inspection body, passed at the end of 1999, laid down a more leisurely timetable than the Bush administration would like.

It says top UN weapons inspector Dr Hans Blix and his team have up to two months from the time they start operating in Iraq to draw up a work programme.

There is no deadline for completing Iraqi disarmament and no particular consequences if it is not completed - only the incentive of UN sanctions being lifted.

So it is obvious why the Bush administration will do everything possible to prevent the inspectors returning to Iraq on the basis of the existing mandate.

Council manoeuvres

In theory, it might be argued that Dr Blix's team already has the legal authority to resume UN inspections for the first time for nearly four years.

The 1999 resolution said it would take over the responsibilities and rights of its earlier incarnation, Unscom.

But it is of course subject to the approval of the Security Council; Dr Blix is expected to brief the council again on what he wants to do after his next meeting with Iraqi officials, due in Vienna on 30 September.

US forces in Turkey: Bush will not allow the military window to close

If the US then objects to the return of the inspectors until a new resolution is passed, it seems politically inconceivable that they could go back.

The question of whether Washington will get the resolution it wants is more open.

The key is the attitude of Russia and France: neither country believes a new resolution is necessary, but neither has said it will veto one if it is presented.

Of the other permanent members, China usually abstains in this kind of situation, while Britain is playing an active part supporting the US behind the scenes and may end up drafting the text.

For the moment, the essential diplomacy is taking place in meetings between the big powers outside UN headquarters.

A draft resolution is not likely until at least some degree of understanding is reached.

Strongest card

The hand of the Bush administration at the UN will be strengthened if Congress acts quickly to pass its own resolution, giving the president the kind of wide-ranging authority to use force that he wants.

Colin Powell: Having no new UN resolution would be a ''recipe for failure''

A few days ago, Iraq's sudden agreement to allow the arms inspectors back unconditionally divided the Security Council and set back Mr Bush's drive for international endorsement of military action.

But Saddam Hussein's message to the UN General Assembly on Thursday then helped the Americans and British, since it suggested that conditions might be set on access to some sites.

Perhaps the strongest card Mr Bush holds is the perception that whether the Security Council backs him or not, he will act against Iraq.

The inevitable has a lot of momentum.