U.S. defends killing Americans who join al Qaeda CHICAGO (Reuters) - The Obama administration asserted on Monday a right to kill Americans overseas who are plotting attacks against the United States, laying out specific details for the first time about a policy that critics argue violates U.S. and international law. U.S. Attorney General Eric Holder said that Americans who have joined al Qaeda or its affiliates can be targeted for lethal strikes if there is an imminent threat to the United States and capturing them is not feasible. In a speech to the Northwestern University School of Law, Holder did not refer directly to the CIA drone strike last year that killed Anwar al-Awlaki, a U.S.-born Muslim cleric who joined al Qaeda's Yemen affiliate and directed many attacks. "Any decision to use lethal force against a United States citizen - even one intent on murdering Americans and who has become an operational leader of al Qaeda in a foreign land - is among the gravest that government leaders can face," he said. "The American people can be - and deserve to be - assured that actions taken in their defense are consistent with their values and their laws," Holder said. Holder received a standing ovation when he entered the crowded auditorium but departed to perfunctory applause as some in the audience expressed surprise by his remarks. A question and answer session was canceled, the event organizers said. Civil liberties groups have decried the program as effectively a green light to assassinate Americans without due process in the courts under the U.S. Constitution, a charge that Holder flatly rejected. Court approval for such strikes was unnecessary, he said, adding "the president may use force abroad against a senior operational leader of a foreign terrorist organization with which the United States is at war - even if that individual happens to be a U.S. citizen." That drew sharp criticism from some in the audience. A third-year law school student, Russell Sherman, called such strikes "assassination." Scott Hiley, a language professor at Northwestern University, said Holder employed "endlessly circular reasoning" to try to explain the policy. President Barack Obama and his aides have fiercely defended their stand on national security in the face of criticism from Republicans in Congress and on the presidential campaign trail that terrorism suspects are treated too leniently. Holder said the use of lethal force against Americans abroad would have to comply with several principles governing the law of war, including ensuring the target was of military value and that steps were taken to limit collateral damage. "The principle of humanity requires us to use weapons that will not inflict unnecessary suffering," he said, but added that "these principles do not forbid the use of stealth or technologically advanced weapons." A U.S. official said that there was fierce debate within the administration about whether Holder should give the speech, questioning if it would assuage or irritate Obama's liberal backers who have been concerned that his policies were too close to that of his predecessor, President George W. Bush. "The targeted killing program raises profound legal and moral questions that should be subjected to public debate, and constitutional questions that should be considered by the judiciary," said Hina Shamsi, director of the American Civil Liberties Union's National Security Project. The United States has launched numerous strikes against al Qaeda leaders in Afghanistan and Pakistan using drones, unmanned aerial vehicles that at times have killed and wounded civilians in addition to the intended target, provoking an outcry. A U.N. special investigator in 2010 called on the United States to halt CIA drone strikes, though the U.N. Secretary General Ban Ki-moon has avoided a direct condemnation and said it was up to governments and military authorities to decide. Holder said the administration abides by "robust oversight" when targeting Americans abroad, informing senior lawmakers about its counterterrorism operations. Like the Bush administration, Holder asserted that conducting counterterrorism operations against al Qaeda, the Taliban and their affiliates was the purview of the presidency, citing 2001 congressional authorization. He said the president has constitutional responsibilities to protect and defend the country. "Military and civilian officials must often make real-time decisions that balance the need to act, the existence of alternative options, the possibility of collateral damage, and other judgments," Holder said. The ACLU last month sued the Obama administration in federal court, demanding that Holder's Justice Department release what the civil liberties group says it believes are legal memoranda justifying targeting Americans overseas using lethal force. Additional reporting by Mark Hosenball in Washington and Lou Charbonneau at the United Nations; Writing by Jeremy Pelofsky; Editing by Howard Goller and Philip Barbara