The White House was on the defensive Tuesday trying to justify its once-secret plan to kill U.S. citizens with ties to terrorist groups after a leaked Justice Department memo made the blueprint public.

The memo outlines an administration policy to use armed drones to strike at U.S. citizens overseas if they are part of an active plot to attack America. The strikes can be carried out even if the United States has no evidence of an imminent attack.

The administration defended the policy Tuesday as "ethical" and "wise." But critics of Obama's aggressive use of drone attacks immediately jumped on the latest revelation.

"The issue is that the Obama administration claims an unchecked authority to target an American citizen," said Steven Aftergood, director of the Federation of American Scientists' Project on Government Secrecy. "Do we believe in due process? Do we believe in checks and balances?"

The Obama administration has kept virtually all the details of its drone program under wraps since launching an unprecedented campaign in Pakistan that targeted al Qaeda. The public airing of its rulebook comes just days before John Brennan, the chief architect of the drone program, heads to Capitol Hill to face a confirmation hearing as Obama's nominee to take over the CIA.

When questioned about the memo Tuesday, White House press secretary Jay Carney declined to divulge details not provided in the document, saying the strikes are "legal, they are ethical and they are wise."

Attorney General Eric Holder joined in the defense of the policy. "Our primary concern is to keep the American people safe, but do so in a way that is consistent with our law and our values," he added.

But some accused the president of applying a double standard. He denounced waterboarding and other enhanced interrogation techniques that critics denounce as torture. But then he agreed to a policy of killing suspected terrorists -- including Americans -- without a trial.

"Lethal force is appropriate when you face an imminent threat," said Dixon Osburn, director of the nonprofit Law and Security Program for Human Rights First. "It should be used when there is no other alternative to prevent harm. This white paper turns that idea on its ear."

The leaked memo was equally vague about who can authorize the targeted killing of a U.S. citizen, saying the "U.S. government may not be aware of all plots as they are developing" and has a "limited window of opportunity within which to strike," so a top official could order the killing of someone believed to be planning a terrorist attack.

Polls show the public widely supports the president's use of drones to carry out a covert war against al Qaeda. And unlike with other Democratic presidents, some of Obama's strongest public approval ratings are in the area of national security.

The 16-page leaked memo is labeled "Lawfulness of a Lethal Operation Directed Against a U.S. Citizen Who Is a Senior Operational Leader of al-Qaeda or An Associated Force." It was written prior to a U.S. drone strike in Yemen that killed Anwar al-Awlaki, a U.S.-born cleric accused of aiding al Qaeda in a plot against the United States.