By Nick Childs BBC Pentagon correspondent

As the United States sends thousands more troops to the Gulf ahead of a possible war with Iraq, the Pentagon insists it is continuing a "deliberate, methodical" build-up.

But it certainly seems to be accelerating, and moving into a new phase.

Until now, the build-up has concentrated on getting heavy ground equipment, supplies, and some key capabilities into place.

Now the first big reinforcements of personnel and air power are getting under way.

Crack troops

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The crack 3rd Infantry Division specialises in desert warfare.

Much of its equipment has already been shipped to the Gulf.

Elements of the division, numbering some 4,000 troops, have also been in Kuwait for training.

Now most of their colleagues are heading for Kuwait, too, in the coming weeks.

It will be the first deployment of a full combat division as part of this build-up.

But the US Marine Corps is also making preparations to move at least one Marine Expeditionary Force of some 17,500 personnel to the region.

The equipment for such a force is already being moved forward from floating store ships at the British Indian Ocean island of Diego Garcia.

Kuwait focus

Much of the build-up is focused on Kuwait, right across Iraq's southern border.

A detachment of Apache helicopters is among the recent reinforcements.

Washington has also deployed army and marine headquarters from the 5th Corps in Germany and the 1st Marine Expeditionary Force in California to Kuwait.

They would command any land campaign.

Several hundred extra specialist personnel from US forces in Germany are also being sent.

Air power

US air power in the region is also getting a boost.

Among the units ordered to deploy are the 28th Bomb Wing of B-1B bombers from Ellsworth air force base in South Dakota, the 4th Fighter Wing of F-15E strike aircraft from North Carolina, and F-15C fighters from Langley air force base in Virginia.

The US is also boosting its force of Predator unmanned drones and other surveillance aircraft.

These forces could be sent to bases in Kuwait, Qatar, Bahrain, Oman, or even Diego Garcia.

Final surge

Privately, Pentagon officials caution against seeing this as a final surge in deployments.

And the signs are that the US military is still pacing its build-up to keep its options open, with more deployments to come.

Analysts suggest that the US would want the equivalent of three or four combat divisions for a full-scale operation, but the 3rd Infantry Division is the only one so far to get its marching orders.

But among the units being deployed are what the Pentagon describes as 'enabling forces,' such as combat support troops, which could speed the arrival of reinforcements.

And one option also open to the Pentagon is what is known as a "rolling start", beginning operations before all the required forces have actually been sent.

Raising the stakes

For the moment, the Bush administration remains anxious to insist that war is not a foregone conclusion.

But it is raising the military stakes, as well adding to the pressure on Baghdad.

Naval build-up

A giant navy hospital ship, the USNS Comfort, is due to set sail on Monday for the area.

The US Navy has also told the aircraft carrier Abraham Lincoln and its battle group, which had been on their way back to the US from the Gulf, to remain in the western Pacific.

Another has been put on short-notice alert, and the Navy has accelerated the training schedules of two others.

And with the two currently deployed in and around the Middle East, that could mean six of these ships could be available in the next few weeks.