The White House has defended drone strikes against Al-Qaeda suspects as legal, ethical and wise and insisted they complied with US law and the Constitution, even if they targeted Americans.
The White House defended President Barack Obama's power to wage drone war after a Justice Department memo argued that Americans high up in Al-Qaeda could be lawfully killed, even if intelligence fails to show them plotting an attack.
The disclosure by NBC news, which posted a link to the white paper on its web page, came as US drone attacks in Pakistan, Yemen and elsewhere face increasing scrutiny and questions from human rights groups.
"We conduct those strikes because they are necessary to mitigate ongoing actual threats, to stop plots, to prevent future attacks and, again, save American lives," said White House spokesman Jay Carney.
"These strikes are legal, they are ethical, and they are wise."
Among the most controversial of the attacks were the September 2011 killings in Yemen of Anwar al-Awlaki and Samir Khan, which stoked concern because the two were US citizens who had never been charged with a crime.
"I would point you to the ample judicial precedent for the idea that someone who takes up arms against the United States in a war against the United States is an enemy and therefore could be targeted accordingly," Carney said.
The white paper offers a more expansive definition of self-defense and imminent attack than those given publicly in the past by senior US officials, who have cited "the inherent right to self-defense" in defending the attacks.
"The condition that an operational leader present an 'imminent' threat of violent attack against the United States does not require the United States to have clear evidence that a specific attack on US persons and interests will take place in the immediate future," the memo says.
Instead, an "informed, high-level" official could decide that the targeted individual posed "an imminent threat of violent attack against the United States" if he had "recently" engaged in such activities, and there was no evidence he had renounced or abandoned them.
The memo also says the individual's capture must be unfeasible, and can be considered so if capture posed an "undue risk" to US personnel.
The 16-page memo is entitled "Lawfulness of a Lethal Operation Directed Against a US Citizen Who Is a Senior Operational Leader of Al-Qaeda or An Associated Force."
NBC said the memo was given to the Senate Intelligence and Judiciary Committees in June on condition it be kept confidential and not discussed publicly.
Its leak comes just two days before White House counter-terrorism chief John Brennan goes before the Senate for hearings on his nomination to be head of the CIA.
Brennan has been a central player in the US drone campaign, which has expanded sharply under President Barack Obama despite qualms about its legality and public outrage in Pakistan over civilian deaths.
Jameel Jaffer, deputy legal director of the American Civil Liberties Union, which has sued to obtain the legal document used to authorize the killing of Awlaki, a radical preacher, called the white paper "chilling."
"According to the white paper, the government has the authority to carry out targeted killings of US citizens without presenting evidence to a judge before the fact or after, and indeed without even acknowledging to the courts or to the public that the authority has been exercised.
"Without saying so explicitly, the government claims the authority to kill American terrorism suspects in secret," he wrote on the ACLU's web site.
Administration officials fiercely defend the drone program as key to the US strategy against Al-Qaeda, in a war against terrorism with no geographic boundaries.
"It's been an important part of our operations against Al-Qaeda, not just in Pakistan, but also in Yemen, in Somalia and I think it ought to continue to be a tool we ought to use where necessary," outgoing US Defense Secretary Leon Panetta said in an interview with AFP last week.
Meanwhile, inside the United States, lawmakers in at least nine states were working on measures to restrict the use of drones in their skies due to worries that the unmanned vehicles could be used to spy on Americans at home.
The American Civil Liberties Union says the states so far are Oregon, California, Montana, Texas, Nebraska, Missouri, North Dakota, Virginia and Florida.