Khalid Sheikh Mohammed is shown in this file photograph during his arrest on March 1, 2003.

US officials cleared the way on Wednesday for a long-delayed trial of the self-proclaimed mastermind of the 9/11 attacks and four alleged plotters, unveiling charges that carry a possible death sentence.

Khalid Sheikh Mohammed and his accused conspirators have been held for years at the US-run prison in Guantanamo Bay, Cuba, while a legal and political battle has played out over how and where to prosecute them.

“The charges allege that the five accused are responsible for the planning and execution of the attacks of September 11, 2001, in New York and Washington DC, and Shanksville, Pa, resulting in the killing of 2 976 people,” the Defense Department said in a statement.

If convicted before a military tribunal, “the five accused could be sentenced to death,” it said.

More than a decade after the attacks that jolted the American pysche, “it is important to see that justice is done,” White House spokesman Jay Carney said.

He said President Barack Obama was still committed to making good on his promise to close the prison at Guantanamo Bay, after legal setbacks and stiff opposition in Congress forced his hand.

The 46-year-old Mohammed, along with Walid bin Attash of Saudi Arabia, Yemen's Ramzi Binalshibh, Pakistan's Ali Abd al-Aziz Ali - also known as Ammar al-Baluchi - and Mustapha Ahmed al-Hawsawi of Saudi Arabia are due to appear in court for arraignment proceedings within 30 days, the Pentagon said.

The joint trial, which could be months away, will be held at the American naval base in Guantanamo Bay, where the US government has set up special military commissions to try terror suspects.

Mohammed, who US officials refer to simply as “KSM,” has been at the centre of a years-old debate over the legal fate of the accused plotters.

After he was captured nine years ago, Mohammed was subjected repeatedly to the simulated drowning technique known as waterboarding that has been widely condemned as torture and other harsh interrogation methods.

His treatment in US custody has raised questions about whether his statements to interrogators will hold up in a trial, but testimony from a former aide may resolve that problem.

Majid Khan, once Mohammed's deputy, has accepted a plea deal with US authorities that will require him to testify against other terror suspects.

After taking office in 2009, Obama initially sought to hold trial for Mohammed and his four accused accomplices in civilian court in New York, just steps from the Ground Zero site where the World Trade Centre's twin towers fell in 2001.

But the proposal sparked criticism and the president's Republican foes in Congress put an end to those plans by blocking the transfer of terror suspects to the United States.

Buck McKeon, the Republican chairman of the House Armed Services Committee, said it was fitting that the accused be tried before a military tribunal.

“It is time for the American people to hear the fullest possible account of the atrocities that these individuals have committed against the United States,” he said. “It is time for judgement to be passed and long-delayed justice to be done.”

Human rights groups have slammed the Guantanamo tribunals as tainted and renewed demands that terror suspects be tried in a federal courts by civilian judges.

“The Obama administration is making a terrible mistake by prosecuting the most important terrorism trials of our time in a second-tier system of justice,” American Civil Liberties Union executive director Anthony Romero said in a statement.

A lawyer for Ali, the Pakistani defendant, said his client would not be facing possible execution if he was being tried in civilian court.

“Because he did not kill or plan to kill, Mr Ali would not be eligible for the death penalty if this case were tried in federal court,” James Connell said.

The military tribunals were created under George Bush's presidency after the 9/11 attacks, with officials arguing that al-Qaeda militants fell into a category that did not suit regular civilian courts.

Procedures for the military tribunals, also known as commissions, have since been modified by the Obama administration to make them more closely resemble civilian courts.

Mohammed and the other accused plotters were charged once before under the Bush era commissions and, now that the system has been revised, had to be formally charged again to clear the way for a trial.

The five are charged with terrorism, hijacking aircraft, conspiracy, murder in violation of the law of war, attacking civilians, attacking civilian objects, intentionally causing serious bodily injury and destruction of property in violation of the law of war. - Sapa-AFP