By Jonathan Marcus BBC defence correspondent

There are clear signs that the United States military is gearing up for a potential Iraqi operation.

Long-standing improvements to military infrastructure in the Gulf region are well under way.

Bush: Saddam is a threat to all civilisation

The production of key munitions is moving into high gear.

President George Bush and his national security team were briefed on Tuesday by senior military commanders on possible war plans against Iraq.

But the White House is insisting that no decisions have yet been taken.

Doubters remain

The Pentagon has not yet decided on how it would set about attacking Iraq.

Saddam Hussein: Under pressure to allow UN inspectors back

All sorts of options seem to be in play, ranging from a re-run of the US build-up that preceded the last Gulf War to a more innovative approach dubbed "inside out" or "Baghdad first" by which the Iraqi capital would be seized and isolated with the intention of decapitating the Iraqi regime.

Not everyone in the senior reaches of the US military is convinced that an invasion of Iraq is necessary or even wise.

Some argue that the existing strategy of containment should be maintained.

But whatever the differences of opinion, the debates and the strategic arguments, preparations are nonetheless under way for a conflict whenever it comes.

Alternative to Saudi

Commercial satellite images of the air base at Al Udeid in Qatar, published this week, show significant improvements to place the base on a war-footing.

There have been runway improvements, additional hard-standing or parking places for aircraft, along with new hardened shelters.

Since March of this year, the US has reportedly moved in communications and computer equipment to Al Udeid that could, at short notice, enable the base to be used as a Combined Air Operations Centre - an alternative command post should the Prince Sultan facility in Saudi Arabia be unusable for political reasons.

At the same time the US defence industry is also moving into high gear.

The Pentagon used some 5,000 satellite-guided bombs or JDAMs during the Afghan air campaign - nearly half its total stockpile.

These are now the air weapon of choice and production is being ramped up to replenish and expand the stockpile over the coming weeks and months.

While special forces units used in Afghanistan rest and re-train, key US reservists are also being called up including elements of army combat units.