About 1,500 U.S. military personnel are bound for the U.S. Navy base in Cuba to build and then guard a prison that will hold hundreds of Al Qaeda and Taliban detainees "in maximum security," a Pentagon spokesman said Sunday.

The whereabouts of the biggest prize — Usama bin Laden — are still unknown, but U.S. and other anti-terrorism coalition officials are beginning to believe he has fled Afghanistan for Pakistan, two members of the Senate Intelligence Committee said Sunday.

The majority of the U.S. troops being sent to Guantanamo Bay, Cuba, are Army military police from Fort Hood, Texas, said Lt. Cmdr. Jeff Davis, a Pentagon spokesman. About 1,000 troops began deploying Sunday, with another 500 expected to go in the coming weeks, Davis said.

Initially, the troops will prepare a section of the base to hold a first batch of fewer than 100 prisoners, but up to 2,000 prisoners may eventually be housed there, Davis said. Gen. Tommy Franks, the head of the military effort in Afghanistan, said Friday that some prisoners are to arrive at Guantanamo within 10 days.

Military personnel are also being sent from Fort Campbell, Ky., Camp Lejeune, N.C. and Norfolk Naval Station, Va., among other bases, Davis said.

Davis said they will be held in "maximum security" conditions, and will be treated in accordance with international standards for military prisoners and have access to Red Cross and other non-governmental organization personnel.

Davis said officials are making plans in light of the Al Qaeda prison riot that left hundreds dead, including CIA officer Johnny "Mike" Spann.

"We are cognizant of the incident that took place in Mazar-e-Sharif," Davis said. "Many of these people have demonstrated their determination to kill others, kill themselves or escape."

No decision has been made whether to hold military tribunals for some of the prisoners at the Navy base, Davis said.

The U.S. Navy base at Guantanamo predates the communist revolution on the island nation. Fidel Castro's government says the base should have been closed and returned to Cuban control decades ago. The base is well-defended and would offer few avenues of escape for prisoners.

More than 300 suspected Taliban or Al Qaeda members were in U.S. custody this weekend, military officials have said. Soldiers were guarding 275 prisoners at the base in Kandahar, 21 at Bagram air base north of Kabul and one in the northern city of Mazar-e-Sharif, Compton said. Another nine prisoners, including American Taliban John Walker Lindh, are being held on the USS Bataan in the Arabian Sea. Afghan and Pakistani authorities are holding thousands more prisoners captured during the fighting.

But the top targets, Al Qaeda leader bin Laden and Taliban supreme leader Mullah Mohammed Omar, continue to elude the coalition hunt.

Sen. John Edwards, D-N.C., who is traveling with other senators in the region, said Sunday that Uzbekistan's military intelligence service believes bin Laden has crossed the border into Pakistan. Uzbekistan, like Pakistan, borders Afghanistan and has been a U.S. ally in the fight against the Taliban and Al Qaeda.

"I fully expect the Pakistanis will do everything they can to help us locate bin Laden," Edwards told Fox News Sunday.

Intelligence Committee Chairman Sen. Bob Graham, D-Fla., said bin Laden and other top officials have probably escaped Afghanistan, but no one is certain.

"Increasingly as our efforts to get them in Afghanistan have been futile, there is a greater sense that they have, in fact, escaped, and are probably in one of those tribal territories just over the border into Pakistan," Graham said from Miami on ABC's This Week.

Top military officials have said they don't know where bin Laden and Omar are.

Bin Laden was thought to be in the Tora Bora region of Afghanistan, but he has not turned up in searches by U.S. and anti-Taliban forces there. Omar was most recently thought to be near Baghran, northwest of Kandahar, but Afghan officials now say they believe he escaped.

The Associated Press contributed to this report.