WASHINGTON (Reuters) - A classified Pentagon assessment shows about one in five detainees released from the U.S. military prison at Guantanamo Bay has joined or is suspected of joining militant groups like al Qaeda, U.S. officials said on Wednesday.

Flags wave above the sign posted at the entrance to Camp Justice, the site of the U.S. war crimes tribunal compound, at Guantanamo Bay U.S. Naval Base in Cuba May 31, 2009. REUTERS/Brennan Linsley/Pool

The disclosure comes amid revelations that former Guantanamo detainees had joined al Qaeda in the Arabian Peninsula -- a Yemen-based group believed to be behind a failed plot to blow up a U.S. passenger jet on Christmas Day.

Under pressure to increase safeguards, President Barack Obama announced on Tuesday that he had suspended the transfer of additional Guantanamo detainees to Yemen, citing the deteriorating security situation in the country.

But Obama said the suspension would not prevent him from closing the prison, which was opened in early 2002 by the Bush administration to house terrorism suspects.

More than 560 detainees from Guantanamo have been released, the vast majority of them by the Bush administration.

An Obama administration official said the White House had received “no information that suggests that any of the detainees transferred by this administration have returned to the fight.”

Six Yemeni detainees were sent home days before the December 25 attempted bombing. There are 198 detainees left at Guantanamo, which once held 750, Pentagon officials said. Among those still being held there, roughly 91 are Yemeni.

The Guantanamo facility has been condemned internationally because detainees were denied due process for years and for harsh interrogations conducted there.

A previous Pentagon assessment last April showed that 14 percent of former detainees had joined or were suspected of joining militant groups, up from 11 percent in December 2008.

The officials, who spoke on condition of anonymity, said the revised Pentagon assessment showed that percentage had grown to about 20 percent.

‘INEXACT SCIENCE’

Pentagon press secretary Geoff Morrell declined to comment on the latest figures, saying they remained classified, but told reporters, “The trend hasn’t reversed itself.”

Morrell said the vetting process for releasing detainees was an “inexact science,” adding: “You know, we are making subjective calls based upon judgment, intelligence. And so there is no foolproof answer in this realm. That’s what makes this so difficult.”

The Obama administration official said steps had been taken to improve detainee reviews.

A special Guantanamo task force was created by Obama “to conduct the thorough work that had not been done before: to review the relevant information about each detainee, including the threat they pose, to determine whether they should be prosecuted, detained, or transferred,” the official said.

Critics have long accused the Pentagon of exaggerating the threat posed by detainees.

“This is more scaremongering,” Clive Stafford-Smith, director of the UK-based legal charity Reprieve, which represents several detainees at the facility.

“If the Pentagon was honest about its numbers, it would publish the names of those who have ‘gone back to the fight’ and the allegations against them. ... Let’s have this discussion in the open and stop deceiving people,” he added.

Obama has encountered various complications in trying to close the Guantanamo facility and has acknowledged he will not be able to meet a self-imposed one-year deadline to close it that he promised when he took office last January.

Just last month, Obama’s aides announced the U.S. government would proceed with buying an Illinois prison and is bolstering security there so a limited number of Guantanamo detainees can be transferred to it.

But Congress has yet to provide the military the authority or funding to transfer inmates to Illinois and Republicans have argued moving them there posed an unnecessary security risk.