NBC News reported Monday it had obtained a copy of the confidential 16-page memo. The case made for targeting U.S. citizens in countries such as Yemen is similar to, but goes beyond, the one laid out by Attorney General Eric Holder and other officials.

"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 U.S. persons and interests will take place in the immediate future," the memo said.

The undated memo, titled "Lawfulness of a Lethal Operation Directed Against a U.S. Citizen who is a Senior Operational Leader of Al-Qaida or An Associated Force," was given to members of the Senate and House intelligence committees. It said citizens can be considered imminent threats if they have recently been involved in violence and their views remain unchanged.

Such killings would be "a legitimate act of national self-defense that would not violate the assassination ban," the memo said.

Jameel Jaffer, deputy legal director of the American Civil Liberties Union, called the memo "a chilling document."

"Basically, it argues that the government has the right to carry out the extrajudicial killing of an American citizen," he added. "It recognizes some limits on the authority it sets out, but the limits are elastic and vaguely defined, and it's easy to see how they could be manipulated."