"Portable buildings have been set up and people are in them as we prepare for the exercise," at Qatar's As-Sayliyah army base, said Major Bill Harrison, a Central Command spokesman.
A total of up to 1,000 US military personnel, including top war planners, are being deployed to Qatar in advance of the exercise, codenamed Internal Look, in which Central Command head General Tommy Franks and the commanders of the Navy, Army and Marine Corps units in the region will take part.
"There are 600 people from Central Command and most of them have already arrived here," Harrison told AFP.
"We are doing all the things you would do when setting up a command centre -- communications, computers, all the administrative items you need," he said.
The war game will begin at an undisclosed date in early December and last 7-10 days. The scenario remains secret.
The deployment of the forward command centre comes as the administration of President George W. Bush is considering using force against Iraq if it refuses to cooperate fully with UN weapons inspectors and disarm totally.
The hardware for the exercise was developed and tested earlier this year in Tampa, Florida, where the Central Command is based, and recently shipped to Qatar.
It has not been decided what will happen to the command center after the war games are over, the spokesman said.
The deployment of a new US headquarters to Qatar marks a new step in the buildup of US forces in the Gulf emirate which could serve as a launchpad for an attack on Iraq.
More than 4,000 US military personnel are in Qatar, mostly at al-Udeid air base south of Doha, which has a 4,500-metre-long (15,000 feet) runway and new hardened shelters to accommodate up to 80 aircraft.
Al-Udeid is the biggest facility used to stock US munitions and materiel in the Gulf.
In the United States, Central Command spokesman Nick Balice acknowledged the US military was prepositioning equipment in the region also as part of the effort "to be prepared for any contingency that could arise" but refused to give any specifics.
The New York Times said the United States had spent more than 100 million dollars to build more than 20 climate-controlled warehouses at As-Sayliyah, warehouses storing hundreds of tanks, Bradley fighting vehicles and other hardware -- enough to equip a US Army brigade.
General Richard Myers, chairman of the US Joint Chiefs of Staff, said September 13 that the deployable headquarters might remain permanently in Qatar, though he stressed that no final decision had been made.
The Central Command is responsible for US forces throughout southwest Asia, an area with a quarter of the world's oil reserves and simmering tensions that spans 25 countries from Egypt to Afghanistan.
Saudi Arabia, which served as a base for US-led coalition forces in the 1991 Gulf war, has indicated it will not allow military strikes against Iraq from its territory even though it hosts more than 5,000 US troops.