* Tribunal's chief prosecutor expects 20, at most, will be tried
* Scaling back comes after conviction of bin Laden driver thrown out
Far fewer prisoners will be tried in the Guantanamo war crimes tribunals than the Obama administration originally planned after a recent court ruling cast doubt on the viability of some charges, the chief prosecutor for the tribunals told Reuters.
U.S. President Barack Obama's Guantanamo Review Task Force had said 36 detainees could be prosecuted, but the tribunal's chief prosecutor put the figure at 20 at most.
The number set by the task force after a review completed in 2010 was "ambitious" in light of a recent court ruling, Army Brigadier General Mark Martins said.
The drastic scaling back of the prosecutions comes after a U.S. appeals court threw out the conviction of Osama bin Laden's former driver, Salim Hamdan, who was found guilty in 2008 of providing material support for terrorism.
The court agreed with defense arguments that material support was not internationally recognized as a war crime when Hamdan worked for bin Laden's motor pool in Afghanistan between 1996 and 2001.
Congress made material support a war crime in a 2006 law underpinning the Guantanamo tribunals, but the appeals court said the law could not be retroactively applied.
The Guantanamo tribunals were established by the Bush administration and revised by the Obama administration to try suspected al Qaeda operatives and their associates on terrorism charges outside regular U.S. civilian and military courts.
More than 100 of the 166 prisoners in the camp have joined a hunger strike to protest the failure to resolve their fate after more than a decade of detention. Detainees have complained of abuse and torture, which the administration denies, while rights activists and international observers have criticized the government's use of the prison.
Obama, who promised in his 2008 election campaign to close the prison, pledged last month to lift a ban imposed on transfers of Guantanamo detainees to Yemen.
By the time the appeals court threw out Hamdan's conviction in October 2012, he had finished his sentence and returned to Yemen. But the ruling dissuaded prosecutors from pursuing cases against other prisoners they had considered charging with providing material support to al Qaeda, Martins said.
He spoke to Reuters on Monday as lawyers, court personnel, journalists and observers traveled to the Guantanamo Bay Naval Base in Cuba for two weeks of pretrial hearings in the pending cases.
MOST SERIOUS CRIMES
Martins said captives who would be prosecuted by the Guantanamo tribunals included the seven whose trials are finished and the six facing pretrial hearings this week and next. He did not identify the handful of other prisoners he still wanted to charge, but said he would concentrate on those linked to the most serious crimes.
Cori Crider, an attorney who works with the rights group Reprieve and represents some Guantanamo prisoners, noted that the number who would ever face trial represented fewer than 3 percent of the nearly 800 prisoners who have been held at Guantanamo over the years.
"This shockingly low figure demonstrates what a terrible mistake Guantanamo has been, and just how many lives have been ruined for no good reason," Crider said. "It is high time President Obama got his act together and delivered on his promise to close this prison."
Pre-trial hearings resumed at Guantanamo on Tuesday in the death penalty case against Abd al Rahim al Nashiri, a Saudi captive accused of directing suicide bombers to ram a boat full of explosives into the side of the USS Cole while the warship was fuelling off Yemen in 2000. Seventeen U.S. sailors were killed in the explosion.
Defense lawyer Rick Kammen questioned whether prosecutors should be allowed to use as evidence statements given to law enforcement officers in Yemen by another alleged al Qaeda operative linked to the Cole bombing. That man, Fahd al Quso, was killed by a U.S. drone strike in Yemen last year and Kammen said allowing FBI agents to read his statements to the court would deprive Nashiri of his right to confront witnesses.
"Can the United States kill witnesses and then still use their hearsay evidence?" Kammen asked.
The judge, Army Colonel James Pohl, said he also wanted to hear testimony from a Guantanamo prison warden before weighing in on whether defense lawyers should be allowed to carry spiral-bound notebooks during meetings with Nashiri.
The spiral notebooks are outlawed because of concerns the metal spirals could be stripped out and used as weapons.