Washington, Mar 6 (IANS/EFE): Attorney General Eric Holder cited the threat from Al Qaeda Monday as he offered the Obama administration's first detailed public defense of its claim that the president has the right to order US citizens killed abroad.

The nation's top law enforcement official addressed the issue in a speech at Northwestern University School of Law in Chicago.

While he declined to address specific cases, Holder's words clearly referred to US-born Muslim cleric Anwar al-Awlaki, killed Sep 30, 2011, by a drone strike in Yemen.

"(A)n operation using lethal force in a foreign country, targeted against a US citizen who is a senior operational leader of Al Qaeda or associated forces, and who is actively engaged in planning to kill Americans, would be lawful" under certain circumstances, the attorney general said.

"First, the US government has determined, after a thorough and careful review, that the individual poses an imminent threat of violent attack against the United States; second, capture is not feasible; and third, the operation would be conducted in a manner consistent with applicable law of war principles," Holder continued.

Civil liberties groups have criticized the Obama administration for refusing to release the Justice Department opinion that provided the legal basis for killing al-Awlaki, who was never charged with any crime in the US.

"Now, it is an unfortunate but undeniable fact that some of the threats we face come from a small number of United States citizens who have decided to commit violent attacks against their own country from abroad," Holder said in Chicago.

"Based on generations-old legal principles and Supreme Court decisions handed down during World War II, as well as during this current conflict, it's clear that United States citizenship alone does not make such individuals immune from being targeted," he said.

The Fifth Amendment of the US Constitution says that no person can "be deprived of life, liberty, or property, without due process of law".

Al-Awlaki, born in New Mexico to Yemeni immigrant parents, spent his teen years in Yemen, but returned to the US for his higher education, earning degrees in engineering and education from Colorado State University and San Diego State University, respectively.

He produced writings and videos urging Muslims to take up arms against the US government and Washington says he eventually assumed a senior operational role in Al Qaeda.

The cleric's 16-year-old son, Abdulrahman al-Awlaki, also a US citizen, died a few weeks after his father in another drone strike in Yemen.