Barack Obama has given the green light to resume military trials of terror suspects detained at Guantánamo Bay, making a sharp departure from his election promises to close the camp and bring America's fight against terrorism back into the remit of civilian law.

The US president lifted a suspension on so-called "military commissions" which he had imposed on his first full day in the White House. By so doing, he permitted the revival of trials conducted by military officers, with a military judge presiding.

Obama also signed an executive order that moved to set into law the already existing practice on Guantánamo of holding detainees indefinitely without charge.

The president sought to sweeten the pill among civil rights and liberal groups of the resumption of two of the most widely criticised aspects of George Bush's war on terror by emphasising that he still wished to see Guantánamo close. When he came into office in January 2009, he repeatedly promised to have the campclosed within one year. It was set up in the wake of 9/11 in 2001 and thereafter the war in Afghanistan.

As another sweetener, he also defied Republican opposition and said he would continue to allow terror suspects to be tried in federal civilian courts, known as Article III courts, on the US mainland. "I strongly believe that the American system of justice is a key part of our arsenal in the war against al-Qaida and its affiliates. We will continue to draw on all aspects of our justice system including Article III courts to ensure that our security and our values are strengthened," he said.

But civil rights groups condemned what they saw as a decision that flew in the face of the president's earlier promises to close the camp. "The irony could not be more pronounced," said Wells Dixon, a lawyer with the Centre for Constitutional Rights (CCR).

"He came into office saying it was one of his national security priorities to close Guantánamo, yet he has now become one of the first presidents to codify a policy of indefinite detention."

Critics see the resumption of the military trials as confirmation that Guantánamo will be closing no time soon. Under the executive orders allowing detention without charge, there is an in-built review process that allows cases to be reconsidered after the first year and then every four years thereafter.

CCR said that was in itself a tacit admission that Guantánamo would remain a place of extra-judicial detention.

Dixon said that the four-year reviews were also, in legal terms, strikingly similar to the so-called combatant status review tribunals set up by the Bush administration in 2005 and struck down by the supreme court as essentially unconstitutional in 2008.

"This is creating a bureaucratic morass that will achieve nothing legally but will ensure Guantánamo remains open," he said.

Some relatives of victims of the September 2001 attacks said that they were also disappointed by the resumption of military trials. Colleen Kelly, whose brother Bill Kelly Jr died in the Twin Towers on 9/11, said that she knew all too well how important it was to protect all people, not just Americans, from the threat of terrorism.

"There are some seriously dangerous people being held in Guantánamo Bay, I think the world understands that.

"But I think there's also a huge opportunity here being missed to show the world that not only does the US talk the talk, we walk the walk also." She said that the past nine years since 9/11 showed that the criminal system of justice had proven to be fully robust enough to deal with difficult terrorism cases. "There have been more than 170 successful anti-terror prosecutions in civilian courts since 9/11, which to me suggests they work."

Some 172 detainees remain in Guantánamo, of whom fewer than 40 have been earmarked for trial in either criminal courts or military commissions.

Before the military commissions were suspended by Obama, only a handful of cases were ever fully processed under them. Most of them involved guilty pleas, such as that of the Australian David Hicks who was returned to Australia after his conviction.

Salim Ahmed Hamdan, Osama bin Laden's driver, was convicted by military tribunal in 2008 and was returned later that year to Yemen to serve the remainder of his five-and-a-half-year sentence.

It is not yet clear how cases will be decided in terms of whether they should be heard in a civilian or military setting. The issue has become a political hot potato, with Republicans strenuously opposing the transfer of terror suspects to criminal courts on US soil.

The revival of the military commissions is likely to stoke speculation surrounding Khalid Sheikh Mohammed, the suspected principal architect of 9/11. The Obama administration had announced that he would go on trial in a criminal court in New York, but the move provoked a storm of protests from across the political spectrum.

Other possible candidates for a military trial include Abd al-Rahim al Nashiri, the alleged mastermind of the 2000 attack on the USS Cole warship in which 17 American sailors were killed.

Guantánamo timeline

11 January 2002 First prisoners arrive at prison camp set up in the US naval base at Guantánamo Bay, Cuba. Dressed in US prison-issue orange jumpsuits, the detainees enter Camp Xray.

21 March 2002 New military tribunals for detainees are announced.

3 July 2003 President George Bush designates six suspected al-Qaida members as eligible for military tribunals, the first since the second world war.

7 July 2003 The Pentagon creates special military panels (Combatant Status Review Tribunals, or CSRTs) to determine "enemy combatant" status.

23 February 2004 Bush administration lays first charges against detainees. Ali Hamza al-Bahlul from Yemen and Ibrahim al-Qosi from Sudan are charged with conspiracy to commit war crimes.

29 March 2005 The CSRT process is completed. In all, 558 detainees complete the process, with 38 judged to be no longer enemy combatants and eligible for release.

15 February 2006 A UN report recommends closure of Guantánamo. Bush speaks in June of his desire to do so.

29 June 2006 The US supreme court rules the military commission system for Guantánamo violates US and international law, and that the Geneva conventions do apply to detainees.

16 November 2008 President-elect Barack Obama declares his intention to close the prison at Guantánamo.

22 January 2009 Obama orders closure of the prison in one year. New military commissions at the prisonare also suspended.

5 January 2010 After admitting in November 2009 that plans to close the prison by January of 2010 had been missed, Obama reaffirms plans to close the camp later in the year.

7 March 2011 In an apparent acknowledgment that the camp won't be shut down any time, Obama lifts a two-year freeze on new military trials there.