"The deployment order was issued and they are moving," said Navy Lt. Cmdr. Jeff Davis, a Defense Department spokesman.
Davis said many of the U.S. forces at the facility would be Army military police from Fort Hood but other troops would come from Fort Campbell, Kentucky, Camp Lejeune, North Carolina, and Norfolk Naval Air Station, Virginia.
About 500 Marines, including a one-star general, boarded planes Sunday at Camp Lejeune in North Carolina for the U.S. base at Guantanamo Bay. They are primarily members of the 2nd Force Service Support Group.
The total number being deployed is about 1,500.
The new security facility will be run by Marine Corps Brig. Gen. Michael Lehnert from Camp Lejeune.
"Our mission is to go down there, hold (the prisoners) and stay until were told to do otherwise," Camp Lejeune base spokesman Maj. Steve Cox said.
An initial 100 al-Qaida and Taliban prisoners from Afghanistan will be housed there, with the number gradually increasing to 2,000, the Pentagon spokesman said.
John Walker Lindh, a 20-year-old Californian who fought for the Taliban in Afghanistan, is among those being held aboard a U.S. vessel.
Davis said the United States was taking precautions against prisoner uprisings like one at a facility in Mazar-i-Sharif, Afghanistan, which led to many deaths, including that of a CIA operative.
"Many of these detainees have demonstrated their determination to kill others, kill themselves or escape, and we're using the necessary amounts of constraints in order to build appropriate facilities for these detainees given what they demonstrated at Mazar-i-Sharif," Davis said.
He said the prison, however, would conform to the requirements of the Geneva Convention and to "international customary law."
"Their treatment and their detention will be humane and the detainees will have access to appropriate nongovernmental organizations such as the International Red Cross," Davis said.
Cuban leader Fidel Castro had been expected tobject to Washington's decision to build a jail at the 45-square mile American base on Cuba's southeastern tip, but two senators who recently visited Havana said he raised no objection.
The U.S. base was founded after Marines landed at Guantanamo Bay in 1898 during the Spanish-American War and, under a 1934 treaty, can only be disbanded by mutual consent or if the U.S. forces pull out voluntarily.
It is well-defended and would offer few avenues of escape for prisoners. Castro's government says the base should have been closed and returned to Cuban control decades ago.
Castro, however, "is not raising objections to the use of Guantanamo," Sen. Arlen Specter, R-Pa., said before he and Sen. Lincoln Chafee, R-R.I., completed a two-day visit.
Late last week, the Pentagon counted 248 battlefield detainees: 225 in the southern Afghanistan city of Kandahar, 14 at Bagram air field near the capital Kabul, eight on the U.S. Navy assault ship Bataan and one in the northern city of Mazar-i-Sharif.
The prisoners, described by Washington as battlefield detainees, have not been charged with crimes. Defense Secretary Donald Rumsfeld said no decision had been reached on how to conduct any military trials authorized by President Bush.
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