GUANTANAMO BAY NAVAL BASE, Cuba — The U.S. military closed a session of a Guantanamo war crimes trial to journalists and other observers Thursday for the presentation of classified evidence — a first for the tribunal system created to prosecute alleged terrorists.

Anyone without a security clearance was forced to leave the courtroom for the testimony of two witnesses for Osama bin Laden's driver, Salim Hamdan. The defendant stayed in the courtroom.

The witnesses were U.S. Army special forces officers Col. Morgan Banks, a psychologist, and Lt. Col. G. John Taylor, an attorney. Officials did not say why their testimony had to be kept secret.

Hamdan, one of 21 Guantanamo prisoners charged so far, faces up to life in prison if convicted of conspiracy and supporting terrorism at the first U.S. war crimes trial since World War II.

His Pentagon-appointed attorney, Navy Lt. Cmdr. Brian Mizer, said that under court rules he was permitted to disclose only that the two witnesses were at the United States' Bagram air base in Afghanistan when Hamdan was taken there by U.S. forces in December 2001.

"It is my hope that the American public will someday hear Mr. Hamdan's defense," Mizer said in an e-mail before the session.

Some witnesses at Hamdan's trial have been identified only by numbers or their initials, and security officers have cut off audio to observers at pretrial hearings of other detainees to conceal classified information.

But Thursday's session marked the first time the Bush administration's military tribunals at this U.S. Navy base in southeast Cuba have taken testimony in secret, according to Air Force Capt. Paula Bissonette, a tribunals spokeswoman.

The chief prosecutor for the tribunals, Army Col. Lawrence Morris, said the government wants to keep the trials as open as possible.

"It is a balance between the goal of openness ... and the need to address some national security concerns," he said.

The prosecution rested its case earlier Thursday following testimony from a Naval Criminal Investigative Service interrogator, Robert McFadden, who said Hamdan swore allegiance to bin Laden.

Hamdan's lawyers argue he was merely a low-level bin Laden employee, and the Yemeni prisoner has denied swearing loyalty to the al-Qaida chief.

The defense team attempted to block McFadden's testimony, arguing his May 2003 interrogation of Hamdan at Guantanamo was conducted under coercive conditions. But the judge, Navy Capt. Keith Allred, rejected those claims in a heavily redacted ruling Thursday.

Allred said confinement at Guantanamo is "undoubtedly an unpleasant, highly regimented experience, with instant rewards or loss of privileges for infractions." But his ruling said the various disciplinary actions against Hamdan did not have any bearing on the interrogation.

Sahr MuhammedAlly, a lawyer with Human Rights First who has been observing Hamdan's trial, said the judge's ruling and the closed session raise doubts about the tribunals' transparency.

"They say it's fair and open, but secrecy affects every part of these proceedings," MuhammedAlly said.

Military prosecutors plan trials for about 80 of the roughly 265 foreign men held at Guantanamo. So far only one detainee has been convicted: Australian David Hicks, who pleaded guilty under an agreement that sent him home to serve a nine-month prison sentence.