U.S. panel authorizes subpoena of Cheney aide

(Corrects paragraphs 1, 2 and 4 to show subpoena would be issued by committee chair, not subcommittee chair)

WASHINGTON, May 6 (Reuters) - A Democratic-led U.S. congressional panel on Tuesday authorized a subpoena of Vice President Dick Cheney's chief of staff in its probe of possible U.S. torture of suspected terrorists.

House of Representatives Judiciary Committee Chairman John Conyers of Michigan was expected to move quickly with the authorization to subpoena David Addington, who the administration maintains is immune from being required to testify.

If Addington refuses to show up, a court fight is likely, but it may not be resolved until after President George W. Bush and Cheney end their terms in January and leave office.

Without debate, a subcommittee of Conyers' panel approved a resolution authorizing him to subpoena Addington.

Cheney spokeswoman Megan Mitchell said: "We have not yet received a subpoena. Once we do, we will review and respond accordingly."

Bush maintains that the United States does not torture, but he has refused to discuss interrogation techniques, saying he does not want to tip off the enemy.

The CIA has acknowledged using a simulated drowning technique known as waterboarding on three terrorism suspects, but says it stopped using that method in 2003.

Waterboarding has been condemned by human rights groups, foreign countries and many U.S. lawmakers as torture.

The subcommittee authorized the subpoena shortly before beginning a hearing on treatment of enemy combatants.

Those who testified were primarily legal and academic figures who discussed what amounts to torture and the scope of the administration's power to establish interrogation methods.

Former Attorney General John Ashcroft has agreed to testify at a yet-to-be scheduled hearing. John Yoo, a former deputy assistant attorney general, also agreed to testify at a later date after facing a possible subpoena.

Bush has invoked executive privilege in rejecting congressional subpoenas for a number of current and former administration officials, many sought as witnesses in the 2006 firing of nine federal prosecutors.

In March, the House Judiciary Committee filed suit in U.S. District Court asking it to direct White House Chief of Staff Josh Bolten to produce subpoenaed documents and order former White House counsel Harriet Miers to comply with a subpoena and testify about the ousted prosecutors.

A ruling is not expected for at least several months.

Cheney's counsel, Kathryn Wheelbarger, in a letter to the subcommittee, argued Addington cannot be required to testify.

"The office of the vice president remains of the view that the courts, to protect the institution of the vice presidency under the Constitution from encroachment by committees of Congress, would recognize that a chief of staff or counsel to the vice president is immune from compulsion to appear before committees of Congress," Wheelbarger wrote. (Editing by David Alexander and Eric Walsh)