WASHINGTON (AP) — An internal Justice Department memo says it is legal for the government to kill U.S. citizens abroad if it believes they are senior al-Qaida leaders continually engaged in operations aimed at killing Americans.

The document provides a legal rationale behind the Obama administration's use of drone strikes against al-Qaida suspects.

The 16-page document says it is lawful to target al-Qaida linked U.S. citizens if they pose an "imminent" threat of violent attack against Americans, and that delaying action against such people would create an unacceptably high risk. Such circumstances may necessitate expanding the concept of imminent threat, the memo says.

"The threat posed by al-Qaida and its associated forces demands a broader concept of imminence in judging when a person continually planning terror attacks presents an imminent threat," the document added.

A September 2011 drone strike in Yemen killed Anwar al-Awlaki and Samir Khan, both U.S. citizens.

The memo does not require the U.S. to have information about a specific imminent attack against the U.S. But it does require that capture of a terrorist suspect not be feasible and that any such lethal operation by the United States targeting a person comply with fundamental law-of-war principles.

"A decision maker determining whether an al-Qaida operational leader presents an imminent threat of violent attack against the United States must take into account that certain members of Al-Qaida ... are continually plotting attacks against the United States" and that "al-Qaida would engage in such attacks regularly to the extent it were able to do so," says the document, which was reported by NBC.

The document also says that a decision maker must take into account that "the U.S. government may not be aware of all al-Qaida plots as they are developing and thus cannot be confident that none is about to occur; and that ... the nation may have a limited window of opportunity within which to strike in a manner that both has a high likelihood of success and reduces the probability of American casualties."

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With this understanding, the document added, a high-level official could conclude, for example, that an individual poses an imminent threat of violent attack against the United States where he is an operational leader of al-Qaida or an associated force and is personally and continually involved in planning terrorist attacks against the United States.

The American Civil Liberties Union said the document is "profoundly disturbing."

"It's hard to believe that it was produced in a democracy built on a system of checks and balances," the ACLU said.

The document says that the use of lethal force would not violate the Fourth Amendment of the Constitution when a targeted person is an operational leader of an enemy force and an informed, high-level government official has determined that he poses an imminent threat of violent attack against the U.S.

The document said the courts have no role to play in the matter.

"Under the circumstances described in this paper, there exists no appropriate judicial forum to evaluate these constitutional considerations. It is well established that 'matters intimately related to foreign policy, and national security are rarely proper subjects for judicial intervention,'" the white paper said.