Seven things to know about Guantanamo

The five senior Taliban figures exchanged for Sergeant Bowe Bergdahl were not among the group of prisoners at the base in southern Cuba who had already been approved for release.

US lawmakers were also angered that they were not given the mandatory 30-days notification of the releases, as the administration said it had to act quickly because Bergdahl's health was failing.

A senior administration official on Thursday said: "There are a significant number of transfers in the pipeline at various stages and I think you're going to be seeing substantial progress this year."

Here is a fact file about the Guantanamo Bay detention center:

It was opened in January 2002 on a US military base on a coastal spit of land in southern Cuba leased from Havana under a treaty dating back to 1903. It was set up in the wake of the 9/11 attacks under the administration of then president George W. Bush to deal with prisoners who became termed "enemy combatants."

But it quickly became a hated symbol of the so-called "war on terror" with orange jumpsuit-clad prisoners photographed shackled and handcuffed in open cages. The cages were later closed.

US President Barack Obama has repeatedly vowed to close the jail, but his attempts have been partly hampered by Congress which refuses to allow any of the inmates to transferred from the prison, dubbed Gitmo, to US soil.

There are currently 149 prisoners in the jail, out of a total of 779 who have been held there over the years. Of the 149, some 78 have been approved for release without charge because they are not considered "a significant security threat to the United States." These include among others 58 Yemenis, five Tunisians, four Afghans and four Syrians.

A second group of 71 remaining prisoners includes 10 who have already been charged and face trial by a special US military commission. A further 23 have been referred for prosecution, while 38 are eligible to have their cases reviewed. The five Taliban who were released last week came from among this group.

Eight detainees have been tried and convicted since the creation of special military commissions in 2006. Six pleaded guilty. Federal authorities reversed the convictions of two of them, while appeals have been filed in two other cases. The four men were transferred back to their home countries.

Only one, Tanzanian national Ahmed al-Ghailani -- who is serving a life sentence for his role in the 1998 attacks on US embassies in Africa -- appeared before a federal court.

The remaining prisoners hail from 19 countries, and include one person described as stateless. In total there are 87 Yemenis, 12 Afghans, 11 Saudis and then a few from such other countries as Egypt, Pakistan and Russia. US efforts to free those who have not been charged have stumbled as the administration has sought third countries to take some of them. Many have been sent back to their home nations, but other countries housing Gitmo detainees include Albania, Algeria, Bermuda and Palau.

US officials say that it costs between $2.7 million to $2.8 million a year to house each prisoner in Guantanamo. That compares to the $78,000 price tag annually for each inmate in a supermax prison within the United States.

The US keeps a "very close eye" on those who have been released. Some figures have estimated that up to 30 percent return to militant groups with the aim of carrying out attacks on Western targets. But a US official said that figure includes both confirmed and suspected cases. He said 16 percent of freed inmates were confirmed to have returned to the battlefield, while about 12 percent are suspected of having done so.

Among those still held in Guantanamo are Khalid Sheikh Mohammed, the self-proclaimed 9/11 mastermind and four others suspected of involvement in the 2001 terror attacks on US soil.