WASHINGTON - The Pentagon is moving forward with plans to build a new, 40-acre detention complex on the main American military base in Afghanistan, officials said, in a stark acknowledgment that the United States is likely to continue to hold prisoners overseas for years to come.

The proposed detention center would replace the cavernous, makeshift American prison on the Bagram military base north of Kabul, which is now typically packed with about 630 prisoners, compared with the 270 held at Guantánamo Bay, Cuba.

Until now, the Bush administration had signaled that it intended to scale back American involvement in detention operations in Afghanistan. It had planned to transfer a large majority of the prisoners to Afghan custody, in an American-financed, high-security prison outside Kabul to be guarded by Afghan soldiers.

But American officials now concede that the new Afghan-run prison cannot absorb all the Afghans now detained by the United States, much less the waves of new prisoners from the escalating fight against Al Qaeda and the Taliban.

The proposal for a new American prison at Bagram underscores the daunting scope and persistence of the United States military’s detention problem, at a time when Bush administration officials continue to say they want to close down the facility at Guantánamo Bay.

Military officials have long been aware of serious problems with the existing detention center in Afghanistan, the Bagram Theater Internment Facility. After the prison was set up in early 2002, it became a primary site for screening prisoners captured in the fighting. Harsh interrogation methods and sleep deprivation were used widely, and two Afghan detainees died there in December 2002, after being repeatedly struck by American soldiers.

Conditions and treatment have improved markedly since then, but hundreds of Afghans and other men are still held in wire-mesh pens surrounded by coils of razor wire. There are only minimal areas for the prisoners to exercise, and kitchen, shower and bathroom space is also inadequate.

Faced with that, American officials said they wanted to replace the Bagram prison, a converted aircraft hangar that still holds some of the decrepit aircraft-repair machinery left by the Soviet troops who occupied the country in the 1980s. In its place the United States will build what officials described as a more modern and humane detention center that would usually accommodate about 600 detainees - or as many as 1,100 in a surge - and cost more than $60 million.

“Our existing theater internment facility is deteriorating,” said Sandra L. Hodgkinson, the senior Pentagon official for detention policy, in a telephone interview. “It was renovated to do a temporary mission. There is a sense that this is the right time to build a new facility.”

American officials also acknowledged that there are serious health risks to detainees and American military personnel who work at the Bagram prison, because of their exposure to heavy metals from the aircraft-repair machinery and asbestos.

“It’s just not suitable,” another Pentagon official said. “At some point, you have to say, ‘That’s it. This place was not made to keep people there indefinitely.’ ”

That point came about six months ago. It became clear to Pentagon officials that the original plan of releasing some Afghan prisoners outright and transferring other detainees to Afghan custody would not come close to emptying the existing detention center.

Although a special Afghan court has been established to prosecute detainees formerly held at Bagram and Guantánamo, American officials have been hesitant to turn over those prisoners they consider most dangerous. In late February the head of detainee operations in Iraq, Maj. Gen. Douglas M. Stone, traveled to Bagram to assess conditions there.

In Iraq, General Stone has encouraged prison officials to build ties to tribal leaders, families and communities, said a Congressional official who has been briefed on the general’s work. As a result, American officials are giving Iraqi detainees job training and engaging them in religious discussions to help prepare them to re-enter Iraqi society.

About 8,000 detainees have been released in Iraq since last September. Fewer than 1 percent of them have been returned to the prison, said Lt. Cmdr. K. C. Marshall, General Stone’s spokesman.

The new detention center at Bagram will incorporate some of the lessons learned by the United States in Iraq. Classrooms will be built for vocational training and religious discussion, and there will be more space for recreation and family visits, officials said. After years of entreaties by the International Committee of the Red Cross, the United States recently began to allow relatives to speak with prisoners at Bagram through video hookups.

“The driving factor behind this is to ensure that in all instances we are giving the highest standards of treatment and care,” said Ms. Hodgkinson, who has briefed Senate and House officials on the construction plans.

The Pentagon is planning to use $60 million in emergency construction funds this fiscal year to build a complex of 6 to 10 semi-permanent structures resembling Quonset huts, each the size of a football field, a Defense Department official said. The structures will have more natural light, and each will have its own recreation area. There will be a half-dozen other buildings for administration, medical care and other purposes, the official said.

The new Bagram compound is expected to be built away from the existing center of operations on the base, on the other side of a long airfield from the headquarters building that now sits almost directly adjacent to the detention center, one military official said.

It will have its own perimeter security wall, and its own perimeter security guards, a change that will increase the number of soldiers required to operate the detention center.

The military plans to request $24 million in fiscal year 2009 and $7.4 million in fiscal year 2010 to pay for educational programs, job training and other parts of what American officials call a reintegration plan. After that, the Pentagon plans to pay about $7 million a year in training and operational costs.

There has been mixed support for the project on Capitol Hill. Two prominent Senate Democrats, Robert C. Byrd of West Virginia and Tim Johnson of South Dakota, have been briefed on the new American-run prison, and have praised the decision to make conditions there more humane.

But the senators, in a May 15 letter to the deputy defense secretary, Gordon England, demanded that the Pentagon explain its long-term plans for detention in Afghanistan and consult the Afghan government on the project.

The population at Bagram began to swell after administration officials halted the flow of prisoners to Guantánamo in September 2004, a cutoff that largely remains in effect. At the same time, the population of detainees at Bagram also began to rise with the resurgence of the Taliban.

Military personnel who know both Bagram and Guantánamo describe the Afghan site, 40 miles north of Kabul, as far more spartan. Bagram prisoners have fewer privileges, less ability to contest their detention and no access to lawyers.

Some detainees have been held without charge for more than five years, officials said. As of April, about 10 juveniles were being held at Bagram, according to a recent American report to a United Nations committee.

Copyright 2008 The New York Times Company