The secrecy shrouding government files on terror suspects is bogging down the Pentagon's effort to hold trials at Guantanamo Bay, with defense attorneys accusing the U.S. government of withholding potential evidence.

At pretrial hearings this week, attorneys for two Al Qaeda suspects captured in Afghanistan said they need more access to interrogators, witnesses and records. Prosecutors objected, citing a need to protect the identities of U.S. service members and other security concerns.

• Guantanamo Bay Detainees Could Face Charges Over 9/11

The hearings did not resolve the disputes, which appear likely to further delay the launch of first U.S. war-crime tribunals since the World War II era. The first detainees were charged more than three years ago, but repeated legal challenges have kept any from going to trial.

"We're going to have to see how willing the judges are to interpret the rules so as to give defense counsel some kind of chance to actually defend their clients," said Navy Lt. Cmdr. William Kuebler, a defense attorney for detainee Omar Khadr. "That means litigating these discovery issues and that takes time."

Trials are scheduled to begin this spring for Khadr, who is accused of hurling a grenade that killed a U.S. soldier in 2002, and Salim Ahmed Hamdan, a former driver for Usama bin Laden who allegedly also delivered weapons for Al Qaeda.

They are minor figures compared with the 15 "high-value" detainees — including Khalid Sheikh Mohammed, the alleged mastermind of the Sept. 11, 2001, terrorist attacks — who are among those expected to face charges. Secrecy may be even a bigger issue in their trials.

The New York Times reported Saturday that military prosecutors are nearing the end of preparations for the "first sweeping case" against as many as six Guantanamo detainees suspected in the Sept. 11 plot — Mohammed likely among them.

The law authorizing the war-crimes tribunals allows the use of classified evidence, and prosecutors say they fulfill their obligation to share it with the other side. But some defense attorneys say the government uses too narrow an interpretation of what information is relevant.

Classified evidence will likely play an increasingly central role as the government forges ahead with plans to prosecute about 80 of the roughly 275 men held at this isolated U.S. Navy base on suspicion of terrorism or links to the Taliban or Al Qaeda.

A Pentagon spokesman, Navy Cmdr. Jeffrey Gordon, said the government's decisions to classify evidence often reflect a need to protect U.S. forces still fighting in Afghanistan.

"The hearings this week demonstrated some of the complexities involved in a new type of war against a new type of enemy," he said, while expressing optimism. "On balance, we're making progress and moving forward."

In Hamdan's case, his attorneys asked the military judge to provide them access to government employees who interrogated Hamdan after his capture in November 2001. One of his attorneys, Charles Swift, said the defense wants to determine whether Hamdan made any statements through coercion.

Hamdan's defense team said they have been provided with only partial, incriminating portions of his interrogation transcripts — an accusation that prosecutors denied.

"Every statement that he has made we have provided," said Army Col. Larry Morris, the chief prosecutor for the military tribunals.

In Khadr's case, Kuebler said the government has refused to put defense lawyers in touch with several eyewitnesses to the 2002 firefight in Afghanistan which Khadr, who was then 15, allegedly hurled a grenade that killed Army Sgt. 1st Class Christopher Speer.

At one of the hearings this week, the government inadvertently released a witness account that raised doubt over whether Khadr threw the grenade. Prosecutors later said they had planned to hand out a redacted version, but Kuebler said he believed the government meant to keep the witness account from the public.

"There's no openness about this process," he said.

The military commissions, as the tribunals are called, convicted one detainee — David Hicks of Australia — but it was through a plea bargain before his trial even began.