The Pentagon plans to double the number of US troops in the Gulf next month, defence officials confirmed. More than 50,000 will flow into the region from early in the new year.

The huge military deployment, expected to be authorised next week by Donald Rumsfeld, the Defence Secretary, is the clearest possible signal that the decision to attack Iraq has all but been made by Mr Bush. Aides say, however, that he will not make a final decision until the last week in January, when the inspections process comes to a head.

On January 27 Hans Blix, the UN’s chief weapons inspector, is scheduled to make his first substantive report to the Security Council on Iraq’s weapons declaration and its co-operation with inspectors. By then the White House intends to have the necessary US and allied troops in place for a military campaign beginning in early February.

“It’s now inevitable war is going to occur,” Michael O’Hanlon, senior fellow at the Brookings Institution, a Washington policy think-tank, said. “The decision is all but made. Once you get a troop deployment of this size, once you get into these types of numbers, around 100,000 troops, war becomes inevitable.

“This is not something the Pentagon can do easily, and probably not more than once.”

Of the American force in the Gulf now, about 15,000 are US Army and Marine ground troops, mostly stationed in Kuwait. The rest are Air Force and Navy personnel. The build-up of US forces expected after the new year will consist of combat forces, maintenance and logistics troops, warships and aircraft. Many would be deployed from US bases in America and Europe.

The Pentagon has enough tanks and armoured personnel carriers to support 55,000 ground troops. With an additional 50,000 troops, the force is still much smaller than the 500,000 troops in the region before the 1991 Gulf War, but the war plan envisages a faster moving, more mobile campaign.

The Iraqis are also girding for conflict. Armoured units of Iraq’s Republican Guard have moved from their garrisons near Baghdad to an area 40 miles west of the capital, the largest deployment by President Saddam Hussein in two years, Pentagon officials said.

The Iraqi military has begun placing obstacles on the runways of air bases in an apparent attempt to prevent allied aircraft landing troops inside the country. The obstacles, including parked lorries and concrete barriers, have been detected by US spy satellites.

The Pentagon has been monitoring activity at bases in western Iraq which would form a likely staging post for troop landings. The airfields have long runways that can accommodate heavy transport vehicles.

In recent weeks Saddam has accelerated imports of spare parts, moved ammunition closer to troop positions and dug trenches for soldiers and military vehicles. Pentagon officials say that Saddam intends to use rivers and other natural features as obstacles to any US advance, and to set up a layered defence with Baghdad at the centre.

There are about 375,000 Iraqi ground troops, down from one million in 1991, and many are poorly trained and equipped. The six Republican Guard divisions, numbering about 90,000, are expected to fight longer. The Iraqi Air Force, which has about 300 fighter jets, is less than half the size it was in 1991.

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