Prosecutors urged a federal appeals court Tuesday to keep terrorism suspect Zacarias Moussaoui (search) from interviewing a senior Al Qaeda operative, arguing he has no constitutional right to question an enemy combatant and disrupt a military interrogation.

"The damage to the United States will be immediate and irreparable," Assistant Attorney General Michael Chertoff argued as he tried to convince the 4th Circuit Court of Appeals (search) that Moussaoui can receive a fair trial without access to the Al Qaeda witness he demands.

The court's ruling could have far-reaching implications, determining whether Moussaoui's case is transferred to a military tribunal and what rights people accused of terrorism in the future will be afforded in the courts.

Moussaoui's lawyers countered that the government cannot carve out an exception to a constitutional right of defendants just because they are charged with terrorism. Moussaoui contends that access to the disputed witness could exonerate him or spare him the death penalty.

Frank Dunham, Moussaoui's court-appointed attorney, told the three-judge panel that if the government refuses access to the witness, it must take some action up to and including dismissal of the charges or removal of the death penalty.

"Declaring a witness unavailable in their custody should not be a freebie," Dunham argued.

Moussaoui, the only U.S. defendant charged as a conspirator with the Sept. 11 hijackers, was not in the courtroom even though he is representing himself. His standby court-appointed lawyers made the case on his behalf even though he is not cooperating with them.

Moussaoui repeatedly has asked for the right to interview several of his Al Qaeda colleagues now in U.S. custody, including suspected Sept. 11 planner Ramzi Binalshibh (search).

U.S. District Judge Leonie Brinkema in Alexandria, Va., granted Moussaoui the right to interview Binalshibh over a closed television hookup after a finding that the prisoner might support Moussaoui's contention that he was not part of the Sept. 11 plot. Moussaoui is accused of being part of the plot in his indictment.

The government appealed.

Tuesday's hearing included a public portion and a private proceeding to protect classified information.

Chertoff, who has been nominated to be a judge on the 3rd U.S. Circuit Court of Appeals, argued in public that the government would suffer immediate damage if Moussaoui was permitted to interrupt military interrogations and gain access to Binalshibh.

Attorney General John Ashcroft filed an affidavit last month that certified the government would not turn over a witness for questioning, but the document was under seal and Chertoff would not discuss it.

Circuit Judge Karen Williams asked him if Moussaoui's request was similar to "graymail," a term used to describe a legal strategy when defendants -- particularly in spy cases -- threaten to expose national secrets to avoid prosecution or penalties.

"This is more than that. It's a blackmail," Chertoff replied. He argued that "the worst kind of defendant" would use this tactic in the future if Moussaoui prevails and the Binalshibh interview is allowed to proceed.

"It could change the course of military activities," Chertoff said.

Dunham argued that the government's objection would enable prosecutors to exclude access to any witnesses undergoing military interrogation.

If a witness can be excluded in such a circumstance, Dunham said, "it doesn't make a difference whether he's in a submarine or in Chicago."

Dunham said the government should suffer legal sanctions if it denies terrorism suspects their right to potentially favorable information. He said the sanctions could include dropping plans to seek the death penalty if Moussaoui is convicted; eliminating from the indictment all references to Binalshibh, who allegedly wired money to Moussaoui, or dismissing the indictment.

Chertoff rejected the alternative of sending the Moussaoui case and future terrorism cases to military tribunals, saying foreign governments might limit the information they provide American prosecutors and refuse to extradite suspects if cases aren't tried in civilian courts.

Moussaoui has asked to interview three additional Al Qaeda figures, including suspected Sept. 11 mastermind Khalid Shaikh Mohammed. Dunham said Brinkema has granted him access only to Binalshibh.

According to documents unsealed by Brinkema and filings in the appellate case, the government considered Moussaoui to be a potential fifth pilot of an airplane that would have crashed into the White House on Sept. 11. He was in U.S. custody at the time.

Moussaoui, an acknowledged Al Qaeda loyalist, has said he was planning to be part of a post-Sept. 11 operation outside the United States.

The appellate judges who heard the arguments were Williams and Circuit Judges William Wilkins and Roger Gregory. They did not indicate when they would rule.