Less than a month after signing an executive order to close the Guantanamo Bay prison camp, President Barack Obama has quietly agreed to keep denying the right to trial to hundreds more terror suspects held at a makeshift camp in Afghanistan that human rights lawyers have dubbed “Obama’s Guantanamo”.

In a single-sentence answer filed with a Washington court, the administration dashed hopes that it would immediately rip up Bush-era policies that have kept more than 600 prisoners in legal limbo and in rudimentary conditions at the Bagram air base, north of Kabul.

Now, human rights groups say they are becoming increasingly concerned that the use of extra-judicial methods in Afghanistan could be extended rather than curtailed under the new US administration. The air base is about to undergo a $60m (£42m) expansion that will double its size, meaning it can house five times as many prisoners as remain at Guantanamo.

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Apart from staff at the International Red Cross, human rights groups and journalists have been barred from Bagram, where former prisoners say they were tortured by being shackled to the ceiling of isolation cells and deprived of sleep.

The base became notorious when two Afghan inmates died after the use of such techniques in 2002, and although treatment and conditions have been improved since then, the Red Cross issued a formal complaint to the US government in 2007 about harsh treatment of some prisoners held in isolation for months.

While the majority of the estimated 600 prisoners are believed to be Afghan, an unknown number – perhaps several dozen – have been picked up from other countries.

One of the detainees who passed through the Afghan prison was Binyam Mohamed, the British resident who is expected to return to the UK this week after his release from Guantanamo Bay. Mr Mohamed’s lawyer, Clive Stafford Smith, head of a legal charity called Reprieve, called President Obama’s strategy “the Bagram bait and switch”, where the administration was trumpeting the closure of a camp housing 242 prisoners, while scaling up the Bagram base to house 1,100 more.

“Guantanamo Bay was a diversionary tactic in the ‘War on Terror’,” said the lawyer. “Totting up the prisoners around the world – held by the US in Iraq, Afghanistan, Djibouti, the prison ships and Diego Garcia, or held by US proxies in Jordan, Egypt and Morocco – the numbers dwarf Guantanamo. There are still perhaps as many as 18,000 people in legal black holes. Mr Obama should perhaps be offered more than a month to get the American house in order. However, this early sally from the administration underlines another message: it is far too early for human rights advocates to stand on the USS Abraham Lincoln and announce, ‘Mission Accomplished’.”

Four non-Afghan detainees at Bagram are fighting a legal case in Washington to be given the same access to the US court system that was granted to the inmates of Guantanamo Bay by a controversial Supreme Court decision last year. The Bush administration was fighting their claim.

Two days into his presidency, Mr Obama promised to shut Guantanamo within a year in an effort to restore America’s moral standing in the world and to prosecute the struggle against terrorism “in a manner that is consistent with our values and our ideals”. But on the same day, the judge in the Bagram case said that the order “indicated significant changes to the government’s approach to the detention, and review of detention, of individuals currently held at Guantanamo Bay” and that “a different approach could impact the court’s analysis of certain issues central to the resolution” of the Bagram cases as well. Judge John Bates asked the new administration if it wanted to “refine” its stance.

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The response, filed by the Department of Justice late on Friday, came as a crushing blow to human rights campaigners. “Having considered the matter, the government adheres to its previously articulated position,” it said.

Tina Foster, executive director of the International Justice Network, the New York human rights organisation representing the detainees, warned last night that “by leaving Bagram open, the administration turns the closure of Guantanamo into essentially a hollow and symbolic gesture”.

She said: “Without reconsidering the underlying policy, which has led to the abuses at Abu Ghraib and the indefinite detention of hundreds of people all these years, then we are simply returning to the status quo. The exact same thing that had the world up in arms has been going on at Bagram since even before Guantanamo.

“People have been tortured to the point that they have died; it is a rallying cry for those who oppose the US actions in Afghanistan; it is not strategic for the US; and, more importantly, holding people indefinitely, regardless of who they are and regardless of the facts, is completely inconsistent with everything we stand for as a country.”

The Department of Justice would only say that the legal briefs in the Washington case “speak for themselves”. It says Bagram is a special case because, unlike Guantanamo, it is sited within a theatre of war.

Mr Obama has pushed out the wider questions about the US policy on detaining terror suspects and supporters of the Taliban in Afghanistan until the summer, ordering a review that will take six months to complete.

The administration is weighing the likely increase in prisoners from an expanded fight against the Taliban in Afghanistan and Pakistan, against the international perception that it is embedding extra-judicial detention into its policies for years to come.