WASHINGTON (Reuters) - Some of them have been locked up for a dozen years. Some are suspected fighters from Yemen, Russia or Pakistan, arrested by U.S. forces in Afghanistan or elsewhere. Several have been linked to al Qaeda.

As the U.S.-led war in Afghanistan winds down, the White House will soon provide Congress a dossier on about 50 non-Afghan detainees in a U.S. military prison north of Kabul. Their uncertain fate presents sensitive security and legal problems for the Obama administration in an echo of Guantanamo Bay.

As with Guantanamo, some prisoners at the Parwan detention center are considered by the U.S. government too dangerous to be freed. Others have unclear links to the Afghan conflict, including a Yemeni arrested in Bangkok and moved to the prison.

The classified dossier could provide U.S. lawmakers a more detailed look at the secretive military detention system and the detainees that have been kept hidden from public view during the nearly 13-year U.S.-led war. And it could raise difficult questions over where and how to resettle them.

If Washington does not, or cannot, keep Parwan open after this year, U.S. officials say prisoners might be detained or tried in the United States, held at sea, repatriated or transferred to third countries, or released when "hostilities" in Afghanistan are over.

When U.S. officials would consider hostilities to be concluded remains unclear.

The fate of wartime detainees in Afghanistan underscores the challenges the White House faces as it struggles to end the wars of the post-September 11 era. It needs to deal with them without exposing itself to criticism that it is jeopardizing U.S. security by freeing dangerous prisoners.

"We're concerned that the administration can move some of these really dangerous guys, frankly without any kind of public knowledge," a House of Representatives aide said on condition of anonymity, referring to the 50 detainees at Parwan.

U.S. lawmakers reacted angrily after the Afghan government released in February 65 Afghan prisoners who had been transferred from U.S. to Afghan custody, despite U.S. warnings that many of the men had clear links to the Taliban.

LIFTING THE VEIL

Lawmakers, in a little-noticed section of a giant defense spending bill passed late last year, required the Pentagon in a classified report due this month to provide biographical and other information on many of the non-Afghan detainees in Parwan.

The report must specify whether detainees at Parwan who are considered to be "enduring security threats" are believed to have taken part in attacks against Americans and provide other information about their suspected militant links and histories.

"The (Defense) Department is aware of our reporting requirements and is working diligently to provide the information required as expeditiously as possible," said Lieutenant Colonel J. Todd Breasseale, a Pentagon spokesman.

Lawmakers also required the Pentagon to issue a declassified version of the new report, but how much of that information will ultimately be made public remains unclear.

The Parwan detainees' identities - and the transfer of some of them in the past to other countries - have remained largely a mystery to the public in the United States and Afghanistan.

Their fate takes on new urgency as foreign troops steadily depart Afghanistan ahead of a year-end deadline to end the U.S.-led mission that began in 2001. The White House has said it might leave a modest force to train Afghan soldiers and pursue al Qaeda if the Afghan government will sign a bilateral deal.

IRAQ FLASHBACK?

For many lawmakers, the situation is reminiscent of Iraq, where U.S. officials handed Ali Mussa Daqduq, a Lebanese-born suspected Hezbollah militant, to the Iraqi government shortly before the U.S. withdrawal in 2011. Officials in Washington were enraged when Daqduq was released in 2012.

The detainees at Parwan, say U.S. and foreign officials, and legal advocates, include more than 30 Pakistanis, two Yemenis, and others with Russian, Tunisian, Jordanian or other nationalities. They have not been charged with any crimes.

According to U.S. court documents, Tunisian Redha al-Najar is believed to have been captured in Pakistan in 2002 and brought to Afghanistan. A Pakistani named Amanatullah was brought there after being captured by British forces in Iraq in 2004 or 2005.

Amin Al-Bakri, a Yemeni merchant, was detained in 2002 in Thailand and brought to Afghanistan, while the other Yemeni, Fadi al-Muqaleh, was a fighter captured in Afghanistan in 2002, an official at the Yemeni Human Rights Ministry said.

U.S. defense officials declined to publicly discuss the detainees' identities, their alleged crimes, or details of the conditions in which they have been held at Parwan.

CONUNDRUM

Another aide in Congress, which has periodically been briefed by U.S. officials about the detainees, said some of the men were "clearly associated" with the Taliban, while others were believed to have links to al Qaeda or other groups.

"There's an effort to wrap our heads around what do we do with these guys," the aide said.

The White House has said it does not plan to send new inmates to Guantanamo Bay, which President Barack Obama has been seeking unsuccessfully to close since he took office.

It may be difficult, however, for the United States to transfer prisoners to Yemen or other countries with a history of prison breaks or release of transferred detainees. Some prisoners have already been repatriated, including a small number sent back to Pakistan in recent years. The United States does not make such transfers public. Eleven of 36 Pakistani prisoners believed held in Afghanistan are being processed for repatriation, said Tasnim Aslam, a spokeswoman for Pakistan's Foreign Office. Others could be repatriated later.

"Most of them were people who were just working there and were arrested. There isn't any hardcore evidence about them being militants," she said.

Returned detainees would be kept under surveillance to make sure they had no militant links, she said. Prisoner advocates there, however, say at least some returned detainees have been held in secret prisons in Pakistan before being released.

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